Post Office returns a large gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'post office returns a large' is the wordplay.
'post office' becomes 'po' (abbreviation).
'returns' says the letters should be written in reverse.
'large' becomes 'L' (eg in clothes sizes).
'po' backwards is 'op'.
'op'+'a'+'l'='OPAL'
(Other definitions for opal that I've seen before include "Quartz-like gemstone" , "One in a ring perhaps" , "Rock" , "Milky white gemstone" , "item in Hatton Garden?" .)