Sinclair played guitar (and occasionally sang) in the root Canterbury band Wilde Flowers before going on to be a founding member of Caravan, switching to bass guitar and sharing lead vocals with Pye Hastings. His compositional output came to the fore on the band's third album, the classic In The Land Of Grey And Pink, on which he wrote and sang the title track, "Golf Girl" and the epic "Winter Wine". Sinclair left Caravan in 1972 to form Hatfield and the North with ex-Delivery members Phil Miller and Pip Pyle, lending his distinctive, quintessentially English voice and increasingly impressive bass playing skills to their two albums, and writing some of their best-loved songs, "Share It", "Let's Eat (Real Soon)" (both with lyrics by Pip Pyle) and "Halfway Between Heaven And Earth". After Hatfield broke up in 1975, Sinclair moved back to Canterbury, starting a carpentry/kitchen-fitting business while maintaining low-key musical activities, often under the humorous moniker Sinclair & The South. He came out of this semi-retirement in 1977 when he was asked by Camel to replace their departed bass player. This stint lasted for two studio albums and half of the live set A Live Record.
In the 1980s, his activities were sporadic. He recorded a collaborative album with Phil Miller and Alan Gowen, Before A Word Is Said, in 1981, reunited with Caravan for the 1982 reunion effort Back To Front, sang on one track of National Health's swansong D.S. Al Coda (also 1982), and joined Phil Miller's In Cahoots, for a residency at the London jazz club the Bull & Gate and, in 1984, a European tour. Sadly, he left before the band undertook its first recordings. His voice or bass were barely heard until the end of the decade, save for a low-key Dutch tour in 1986 and a guest spot of Phil Miller's album Split Seconds (1989). In 1990, there was a one-off reformation of Hatfield and the North and a long-term one of the original Caravan line-up in 1990-91.
At this point, Sinclair formed his own group Caravan Of Dreams, with ex-Camel drummer Andy Ward and former Hatfield roadie Rick Biddulph on bass (live gigs only), plus occasional participation from cousin Dave Sinclair and sax/flautist Jimmy Hastings. The project's eponymous album came out in 1992. Sinclair's next effort, R.S.V.P. (1994), was recorded with a fluctuating line-up including Pip Pyle, Tony Coe and former Happy The Man keyboardist Kit Watkins. Regular touring stopped in 1996 as Sinclair moved to the Netherlands for a few years. He reappeared in 2002 with occasional concerts and archival live releases, but the most exposure came with the reunion of Hatfield and the North in 2005-06, which came to an abrupt end when Pip Pyle died in August 2006. Shortly after that, he left his longtime Canterbury home to move permanently to Italy.