Fish seemed to be off (5)
I believe the answer is:
smelt
'fish' is the definition.
(smelt is a kind of fish)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for smelt that I've seen before include "half a guinea" , "Extract (metal) from ore by heat" , "were high" , "Produce metal by heat" , "Inhaled odour" .)