Grand painting, perhaps, has fellows in costume (7)
I believe the answer is:
garment
'costume' is the definition.
(thesaurus)
'grand painting perhaps has fellows' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'painting perhaps' becomes 'art' (painting is a kind of art).
'has' indicates putting letters inside.
'fellows' becomes 'men' (I've seen this before).
'art' placed around 'men' is 'arment'.
'g'+'arment'='GARMENT'
'in' is the link.
(Other definitions for garment that I've seen before include "Perhaps shift" , "Singular outfit" , "Covering" , "stole, perhaps" , "Piece of clothing" .)