Fish detected by its odour? (5)
I believe the answer is:
'fish' is the definition.
(smelt is a kind of fish)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for smelt that I've seen before include "Food fish often fried", "23 [FISH]", "was whiffy", "were high", "Produce metal by heat".)