Former partner on English river (3)
I believe the answer is:
exe
'english river' is the definition.
(I've seen this before)
'former partner on' is the wordplay.
'former' becomes 'ex'.
'partner' becomes 'e' (**).
'on' is a charade indicator (letters next to each other).
'ex'+'e'='EXE'
(Other definitions for exe that I've seen before include "West Country river" , "River of southwest England" , "Tiverton's river" , "River of SW England" , "Devon river" .)