Fish brings rook into quarrel (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'rook into quarrel' is the wordplay.
'rook' becomes 'r' (abbreviation in chess).
'into' indicates putting letters inside.
'quarrel' becomes 'spat' (spatting is a kind of quarrelling).
'r' going inside 'spat' is 'SPRAT'.
'brings' acts as a link.
(Other definitions for sprat that I've seen before include "chap on a fat-free diet" , "Couple sharing meat" , "chap giving richer course material to the Dutch" , "Type of herring - small person or thing" , "Small food fish" .)