Extremely sinkable claret served up for a celebratory meal (5)
I believe the answer is:
seder
'a celebratory meal' is the definition.
(I've seen this in another clue)
'extremely sinkable claret served up' is the wordplay.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'claret' becomes 'red' (red wine).
'served up' shows that the letters should be reversed in order (in a down clue, letters go up).
'sinkable' with its middle taken out is 'se'.
'red' back-to-front is 'der'.
'se'+'der'='SEDER'
'for' acts as a link.
(Other definitions for seder that I've seen before include "religious ritual" , "Jewish festival" , "religious service" , "Jewish meal" , "religious ceremony" .)