One smoker, terribly irritating (7)
I believe the answer is:
irksome
'irritating' is the definition.
(I've seen this in another clue)
'one smoker terribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'terribly' is an anagram indicator.
'smoker' is an anagram of 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Burdensome and tedious" , "Annoying or tiresome" , "bugging" , "Annoying - tedious" , "Bothersome, annoying" .)