What's clearly smelt with head cut off, or another fish (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tench that I've seen before include "Swimmer" , "Fish of the carp family" , "Coarse fish" , "One in school" , "catch" .)