Sully isn't after a back seat (5)
I believe the answer is:
taint
'sully' is the definition.
(I know that sully can be written as taint)
'isn't after a back seat' is the wordplay.
'isn't' becomes 'aint' (non-standard version of 'isn't').
'after' is a charade indicator (letters next to each other).
'a back seat' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'aint' put after 't' is 'TAINT'.
Can you help me to learn more?
(Other definitions for taint that I've seen before include "(Trace of) corruption" , "Contaminating influence" , "Tarnish" , "Stigmatise" , "Adulterate" .)