One smoker reformed is tedious (7)
I believe the answer is:
irksome
'tedious' is the definition.
(synonyms)
'one smoker reformed' is the wordplay.
'one' becomes 'I' (Roman numeral).
'reformed' indicates anagramming the letters.
'smoker' anagrammed gives 'rksome'.
'i'+'rksome'='IRKSOME'
'is' is the link.
(Other definitions for irksome that I've seen before include "Tiresome, irritating" , "Troublesome, vexing" , "Bothersome, annoying" , "Smoker I find annoying" , "Smokier (anag.)" .)