Mate’s ship evacuated on northern river (6)
I believe the answer is:
spouse
'mate's' is the definition.
(mate can mean one's spouse)
'ship evacuated on northern river' is the wordplay.
'evacuated' means to remove the middle letters (internal letters are evacuated from the word).
'on' says to put letters next to each other.
'northern river' becomes 'ouse' (Yorkshire river. I am not sure about the 'northern' bit.).
'ship' with its middle removed is 'sp'.
'sp'+'ouse'='SPOUSE'
Can you help me to learn more?
(Other definitions for spouse that I've seen before include "Consort" , "Husband or wife" , "Very hot and dry" , "Possibly wife" , "Partner in marriage" .)