Constant smoker's Greek character touring north (4)
I believe the answer is:
etna
'constant smoker's' is the definition.
(I've seen this before)
'greek character touring north' is the wordplay.
'greek character' becomes 'eta' (letter from the Greek alphabet).
'touring' is an insertion indicator (tour can mean to go around).
'north' becomes 'N' (abbreviation).
'eta' enclosing 'n' is 'ETNA'.
(Other definitions for etna that I've seen before include "Constant smoker" , "Long-term smoker" , "Threatening mountain" , "Explosive type" , "Sicily's 15" .)