Mum grabbed by sailor in river (5)
I believe the answer is:
tamar
'river' is the definition.
(I've seen this in other clues)
'mum grabbed by sailor' is the wordplay.
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'grabbed by' is an insertion indicator.
'sailor' becomes 'tar' (tar is a kind of sailor).
'ma' placed inside 'tar' is 'TAMAR'.
'in' acts as a link.
(Other definitions for tamar that I've seen before include "River in south-west" , "River of SW England" , "Plymouth river" , "River and bridge which connect Devon and Cornwall" , "Devon-Cornwall boundary river" .)