One smoker dreadfully annoying (7)
I believe the answer is:
irksome
'annoying' is the definition.
(I've seen this before)
'one smoker dreadfully' is the wordplay.
'one' becomes 'i' (Roman numeral).
'dreadfully' indicates anagramming the letters.
'smoker' with letters rearranged gives 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Bothersome, annoying" , "'Tedious, tiresome (7)'" , "Smokier (anag.)" , "Annoying or tiresome" , "Tediously annoying" .)