Smells bit of fish turning, fish oddly (5)
I believe the answer is:
niffs
'smells' is the definition.
(niff is a kind of smell)
'bit of fish turning fish oddly' is the wordplay.
'bit of fish' becomes 'fin' ('fin' is part of 'fish').
'turning' says the letters should be written in reverse.
'oddly' means one should take alternating letters.
The alternate letters of 'fish' are 'fs'.
'fin' written backwards gives 'nif'.
'nif'+'fs'='NIFFS'
(Other definitions for niffs that I've seen before include "Pongs" , "Honks" .)