Mummy’s boy’s lost with naughty child (3)
I believe the answer is:
imp
'naughty child' is the definition.
(I've seen this before)
'mummy's boy's lost with' is the wordplay.
'mummy's boy' becomes 'wimp' (I've seen this before).
'lost' suggests deleting specific letters.
'with' becomes 'w' (abbreviation).
'wimp' with 'w' taken away is 'IMP'.
(Other definitions for imp that I've seen before include "old-fashioned graft" , "Small demon" , "Sprite; add feathers to" , "Minx" , "one likely to cause trouble" .)