One smoker horribly irritating (7)
I believe the answer is:
irksome
'irritating' is the definition.
(I've seen this in another clue)
'one smoker horribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'horribly' indicates an anagram (a horribly wrong version of the letters).
'smoker' is an anagram of 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Burdensome and tedious" , "Smoker I find annoying" , "bugging" , "'Annoying, irritating or boring (7)'" , "Tediously annoying" .)