Mock mum in the morning (4)
I believe the answer is:
sham
'mock' is the definition.
(both can mean fake)
'mum in the morning' is the wordplay.
'mum' becomes 'sh' (both can mean 'keep quiet').
'in the morning' becomes 'am' (AM on a clock).
'sh'+'am'='SHAM'
(Other definitions for sham that I've seen before include "imitation" , "Pretense; bogus" , "Phoney, bogus" , "charade" , "Pretence; bogus, false" .)