Charlatan in Post Office regrets returning (6)
I believe the answer is:
poseur
'charlatan' is the definition.
(I've seen this in another clue)
'post office regrets returning' is the wordplay.
'post office' becomes 'po' (abbreviation).
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'returning' is a reversal indicator.
'rues' reversed gives 'seur'.
'po'+'seur'='POSEUR'
'in' is the link.
(Other definitions for poseur that I've seen before include "Person habitually pretending to be something he is not" , "Insincere show-off" , "Pretentious type" , "Pretentious person" , "One who strikes attitudes, a phoney" .)