Mum in highest part of ship (7)
I believe the answer is:
maintop
'part of ship' is the definition.
(I've seen this before)
'mum in highest' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'highest' becomes 'top' (at the top).
'ma'+'in'+'top'='MAINTOP'
(Other definitions for maintop that I've seen before include "Platform on a mast of a square-rigger - I'm on tap (anag)" , "Sail" , "one such at sea" , "Part of ship" , "Superior position on board" .)