Mum in ship suffocates (8)
I believe the answer is:
smothers
'suffocates' is the definition.
('smother' can be a synonym of 'suffocate')
'mum in ship' is the wordplay.
'mum' becomes 'mother' (I've seen this before).
'in' is an insertion indicator.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'mother' inserted into 'ss' is 'SMOTHERS'.
(Other definitions for smothers that I've seen before include "Suffocates, stifles" , "envelops" , "Makes breathless" .)