Pants erupt with smell becoming foul (10)
I believe the answer is:
putrescent
'foul' is the definition.
(thesaurus)
'pants erupt with smell' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'with' is a charade indicator (letters next to each other).
'smell' becomes 'scent' (scent is a kind of smell).
'erupt' is an anagram of 'putre'.
'putre'+'scent'='PUTRESCENT'
'becoming' acts as a link.
(Other definitions for putrescent that I've seen before include "Decaying, rotting" , "High" .)