Veteran, say, back in lead (6)
I believe the answer is:
stager
'veteran' is the definition.
(I've seen this before)
'say back in lead' is the wordplay.
'say' becomes 'eg'.
'back' is a reversal indicator.
'in' is an insertion indicator.
'lead' becomes 'star' (synonyms).
'eg' written backwards gives 'ge'.
'ge' put into 'star' is 'STAGER'.
(Other definitions for stager that I've seen before include "experienced performer" , "Greats (anag.)" , "old actor" , "Old hand" , "(Old?) person of much experience" .)