One miserable smoker is irritating (7)
I believe the answer is:
irksome
'is irritating' is the definition.
(thesaurus)
'one miserable smoker' is the wordplay.
'one' becomes 'i' (Roman numeral).
'miserable' is an anagram indicator (I've seen this in other clues).
'smoker' is an anagram of 'rksome'.
'i'+'rksome'='IRKSOME'
(Other definitions for irksome that I've seen before include "Bothersome, annoying" , "bugging" , "'Annoying, irritating or boring (7)'" , "Tiresome, irritating" , "Tediously annoying" .)