Claim to express contentment about Post Office sorting centre (7)
I believe the answer is:
purport
'claim' is the definition.
(purporting is a kind of claiming)
'to express contentment about post office sorting centre' is the wordplay.
'to express contentment' becomes 'purr' (I've seen this before).
'about' means one lot of letters goes inside another.
'post office' becomes 'po'.
'centre' indicates the central letters.
The middle letter of 'sorting' is 't'.
'purr' placed around 'po' is 'purpor'.
'purpor'+'t'='PURPORT'
(Other definitions for purport that I've seen before include "Pretend; meaning" , "Meaning, substance" , "Spirit" , "Seem or claim (to be)" , "Intended meaning" .)