Catch one of former pair in river (3)
I believe the answer is:
exe
'river' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for exe that I've seen before include "Topsham's river" , "West Country river" , "Tiverton's river" , "River of southwest England" , "Banker" .)