River where mum takes Jack out (5)
I believe the answer is:
tamar
'river' is the definition.
(I've seen this before)
'mum takes jack out' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'takes' is an insertion indicator (take can mean to capture or invade).
'jack out' becomes 'tar' (both are informal terms for sailor. I am not sure about the 'out' bit.).
'ma' inserted into 'tar' is 'TAMAR'.
'where' is the link.
Can you help me to learn more?
(Other definitions for tamar that I've seen before include "River forming West Country boundary" , "River and bridge which connect Devon and Cornwall" , "River of SW_England" , "River of Devon-Cornwall border" , "River in south-west" .)