One finds smoker terribly annoying (7)
I believe the answer is:
irksome
'annoying' is the definition.
(I've seen this before)
'one finds smoker terribly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'finds' says to put letters next to each other.
'terribly' is an anagram indicator.
'smoker' with letters rearranged gives 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Annoying or tiresome" , "Annoying - tedious" , "Smokier (anag.)" , "bugging" , "Bothersome, annoying" .)