Gainsbourg's stuff? (5)
I believe the answer is:
serge
'stuff?' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for serge that I've seen before include "Strong twilled worsted fabric" , "Twilled fabric; name" , "Twilled woollen fabric" , "Stuff" , "Frenchman" .)