Caught a whiff of sulphur (or fish) (6)

I believe the answer is:

smelts

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'fish' is the definition.
(I've seen this before)

'caught a whiff of sulphur' is the wordplay.
'caught a whiff' becomes 'smelt' (to smell something is to catch a whiff of it).
'of sulphur' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'smelt'+'s'='SMELTS'

'or' is the link.

Can you help me to learn more?

(Other definitions for smelts that I've seen before include "Extracts metals by heating" , "Fish" , "Extracts metal by heating" .)

I've seen this clue in The Independent.

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