Rod broken by fish? Admit defeat (4,3)
I believe the answer is:
drop out
'admit defeat' is the definition.
('drop out' can be a synonym of 'admit defeat')
'rod broken by fish?' is the wordplay.
'broken' indicates anagramming the letters.
'by' means one lot of letters go next to another.
'fish?' becomes 'pout' (pout is a kind of fish**).
'rod' anagrammed gives 'dro'.
'dro'+'pout'='DROP-OUT'
(Other definitions for drop out that I've seen before include "One who withdraws" , "withdrawn student" , "twenty-two" , "leave society?" , "Withdraw from activity" .)