… alas, mostly fake (4)
I believe the answer is:
sham
'fake' is the definition.
(sham can mean fake or pretend)
'alas mostly' is the wordplay.
'alas' becomes 'shame' ('alas' can mean 'that's a 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; bogus, false" , "imitation" , "False and phoney" , "Mash, being false" , "Unreal" .)