Smoker's French and doesn't apply (4)
I believe the answer is:
etna
'smoker's' is the definition.
(volcano Mount Etna, something which 'smokes')
'french and doesn't apply' is the wordplay.
'french and' becomes 'et' ('and' in the French language).
'doesn't apply' becomes 'na' (abbreviation for not applicable).
'et'+'na'='ETNA'
(Other definitions for etna that I've seen before include "Neat (anag)" , "source of fumes?" , "Impressive mount" , "A feature of Sicily" , "one blowing top?" .)