Mostly, or possibly all, grey (7)
I believe the answer is:
largely
'mostly' is the definition.
('largely' can be a synonym of 'mostly')
'possibly all grey' is the wordplay.
'possibly' indicates an anagram.
'all'+'grey'='allgrey'
'allgrey' anagrammed gives 'LARGELY'.
'or' is the link.
(Other definitions for largely that I've seen before include "Mostly found in gallery" , "Fro the most part" , "to a considerable extent" , "On the whole" , "as a rule" .)