Briefly a way in for this painter (6)
I believe the answer is:
ingres
'for this painter' is the definition.
(I've seen this before)
'briefly a way in' is the wordplay.
'briefly' means to remove the last letter.
'a way in' becomes 'ingress' (I've seen this before).
'ingress' with its final letter taken off is 'INGRES'.
(Other definitions for ingres that I've seen before include "French painter known for his portraits" , "French artist" , "Jean -; Fr. artist; singer (anag.)" , "French painter; singer (anag)" , "Producer of old pictures" .)