One reformed smoker irritating (7)
I believe the answer is:
irksome
'irritating' is the definition.
(I've seen this before)
'one reformed smoker' is the wordplay.
'one' becomes 'I' (Roman numeral).
'reformed' indicates anagramming the letters.
'smoker' anagrammed gives 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Smokier (anag.)" , "'Annoying, irritating or boring (7)'" , "bugging" , "Tiresome, irritating" , "Annoying or tiresome" .)