Fake post office keeps used letters (6)
I believe the answer is:
pseudo
'fake' is the definition.
(I know that fake can be written as pseudo)
'post office keeps used letters' is the wordplay.
'post office' becomes 'po' (abbreviation).
'keeps' indicates putting letters inside (inserted letters are kept inside).
'letters' indicates an anagram.
'used' anagrammed gives 'seud'.
'po' enclosing 'seud' is 'PSEUDO'.
(Other definitions for pseudo that I've seen before include "Prefix for fake" , "O dupes! being false and insincere" , "Use pod (anag.)" , "Counterfeit" , "Insincere person" .)