Posh claret, on coaster, is aged (7)
I believe the answer is:
matured
'is aged' is the definition.
(I've seen this in another clue)
'posh claret on coaster' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'claret' becomes 'red' (red wine).
'on' is a charade indicator (letters next to each other).
'coaster' becomes 'mat ' (I've seen this before).
'u'+'red'='ured'
'ured' put after 'mat' is 'MATURED'.
(Other definitions for matured that I've seen before include "Became fully ripe" , "Developed fully" , "Developed; grew up" , "Mellowed" , "Ripened" .)