Not genuine, but used, oddly, in Post Office (6)
I believe the answer is:
pseudo
'not genuine' is the definition.
(I've seen this before)
'used oddly in post office' is the wordplay.
'oddly' is an anagram indicator.
'in' indicates putting letters inside.
'post office' becomes 'po'.
'used' is an anagram of 'seud'.
'seud' placed into 'po' is 'PSEUDO'.
'but' is the link.
(Other definitions for pseudo that I've seen before include "Counterfeit" , "'Fake, sham (6)'" , "Prefix for fake" , "artificial" , "Does up, looking phoney" .)