Pity, for the most part, is pretence (4)
I believe the answer is:
sham
'pretence' is the definition.
(I've seen this before)
'pity for the most part' is the wordplay.
'pity' becomes 'shame' ('shame' can be a synonym of 'pity').
'for the most part' means to remove the last letter.
'shame' with its last letter taken off is 'SHAM'.
'is' is the link.
(Other definitions for sham that I've seen before include "Cheap falseness" , "Pretence, hoax" , "Mash, being counterfeit" , "Not what it seems to be" , "Pretense; bogus" .)