Humiliation? Mostly pretence (4)
I believe the answer is:
sham
'pretence' is the definition.
(I've seen this before)
'humiliation? mostly' is the wordplay.
'humiliation?' becomes 'shame' (humiliation is a kind of shame).
'mostly' means to remove the last letter.
'shame' with its last letter taken away is 'SHAM'.
(Other definitions for sham that I've seen before include "Pretence, hoax" , "imitation" , "'Fake, pseudo (4)'" , "Pretence; bogus, false" , "Phoney kind of hams" .)