Save last of port in cask (4)
I believe the answer is:
butt
'cask' is the definition.
(I've seen this before)
'save last of port in' is the wordplay.
'save' becomes 'but' (I've seen this before).
'last of' says to take the final letters.
'in' indicates putting letters inside.
The final letter of 'port' is 't'.
'but' going around 't' is 'BUTT'.
(Other definitions for butt that I've seen before include "What goat may do" , "couple of hogsheads?" , "End of stock" , "Barrel - laughing stock" , "Strike with head" .)