Support retro vehicle used by writer (9)
I believe the answer is:
subscribe
'support' is the definition.
(I know that support can be written as subscribe)
'retro vehicle used by writer' is the wordplay.
'retro' says the letters should be written in reverse (retro is Latin for 'backwards').
'vehicle' becomes 'bus' (public transport vehicle).
'used by' means one lot of letters go next to another.
'writer' becomes 'scribe' (scribe is a kind of writer).
'bus' back-to-front is 'sub'.
'sub'+'scribe'='SUBSCRIBE'
(Other definitions for subscribe that I've seen before include "Receive or obtain regularly" , "Receive on a regular basis, magazine say" , "Agree" , "Sign up" , "Give regular payments" .)