A French painter puts it up en masse (7)
I believe the answer is:
matisse
'a french painter' is the definition.
(I've seen this before)
'it up en masse' is the wordplay.
'up' indicates an anagram.
'en' indicates putting letters inside (French for \'in\').
'it' anagrammed gives 'ti'.
'ti' put within 'masse' is 'MATISSE'.
'puts' is the link.
(Other definitions for matisse that I've seen before include "Henri -, C twenty Fr. painter, sculptor, illustrator" , "Henri -, nineteenth-twentieth century French painter/sculptor" , "French painter, d. 1954" , "French painter, Henri, d.1954" , "Henri --, French artist" .)