Open posh prawn cocktail (6)
I believe the answer is:
unwrap
'open' is the definition.
(I've seen this before)
'posh prawn cocktail' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'cocktail' indicates an anagram.
'prawn' anagrammed gives 'nwrap'.
'u'+'nwrap'='UNWRAP'
(Other definitions for unwrap that I've seen before include "peel" , "Open (a present)" , "Take the covering off, a parcel say" , "Remove the covering from" , "Remove the outer cover" .)