Breathe in very noisily after foolish sin (5)
I believe the answer is:
sniff
'breathe in' is the definition.
(sniffing is a kind of breathing in)
'very noisily after foolish sin' is the wordplay.
'very noisily' becomes 'ff'.
'after' is a charade indicator (letters next to each other).
'foolish' is an anagram indicator (a foolish spelling).
'sin' anagrammed gives 'sni'.
'ff' after 'sni' is 'SNIFF'.
(Other definitions for sniff that I've seen before include "Slight suspicion (perceived though the nose?)" , "What tracker dogs do" , "Smell at" , "Inhale (glue); slight suspicion" , "Chance" .)