Minor argument about head of rotten fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(I know that sprat is a type of herring)
'minor argument about head of rotten' is the wordplay.
'minor argument' becomes 'spat' (I've seen this in another clue).
'about' is an insertion indicator.
'head of' suggests taking the first letters.
The first letter of 'rotten' is 'r'.
'spat' going around 'r' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Traps for small fish" , "Small fish (used to catch mackerel?)" , "'Set a . . . . . to catch a whale'" , "Traps (anag.)" , "Herring of the east North Atlantic" .)