Unhappily married to office? I don’t believe you! (4,3,2)
I believe the answer is:
come off it
'i don't believe you' is the definition.
('come off it' is an expression of disbelief)
'unhappily married to office?' is the wordplay.
'unhappily' is an anagram indicator.
'married' becomes 'm'.
'm'+'to'+'office'='mtooffice'
'mtooffice' with letters rearranged gives 'COME OFF IT!'.
(Other definitions for come off it that I've seen before include "Stop messing about!" , "Stop being so stupid!" , "that's most unlikely" , "Don't make me laugh" , "Drug addict should" .)