Pants all grey except for small part (7)
I believe the answer is:
largely
'except for small part' is the definition.
I can't tell whether this defines the answer.
'pants all grey' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'all'+'grey'='allgrey'
'allgrey' with letters rearranged gives 'LARGELY'.
Can you help me to learn more?
(Other definitions for largely that I've seen before include "Mostly found in gallery" , "Mainly" , "Generally" , "On the whole" , "to a considerable extent" .)