Fish detected by its odour? (5)
I believe the answer is:
smelt
'fish' is the definition.
(smelt is a kind of fish)
I can't explain the remainder of the clue.
(Other definitions for smelt that I've seen before include "23 [FISH]" , "whiffed" , "Fish related to the salmon (which ponged?)" , "Stank" , "Food fish often fried" .)