Mum coming west is replacing bathing suit (8,7)
I believe the answer is:
swimming costume
'bathing suit' is the definition.
(synonyms)
'mum coming west is replacing' is the wordplay.
'replacing' indicates an anagram (placing again).
'mum'+'coming'+'west'+'is'='mumcomingwestis'
'mumcomingwestis' is an anagram of 'SWIMMING-COSTUME'.
(Other definitions for swimming costume that I've seen before include "Item of beachwear" , "bathers" , "going on in baths?" , "skimpy garment" .)