Job, a bit of a headache, is tedious (5)
I believe the answer is:
heist
'job' is the definition.
(thesaurus)
'a bit of a headache is tedious' is the wordplay.
'a bit of' indicates a hidden word (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'HEIST' can be found hidden inside 'headache is tedious'.
(Other definitions for heist that I've seen before include "Armed hold-up (sl.)" , "Armed robbery" , "Mugging" , "Robbery (mainly US)" , "crime?" .)