Pig enjoys first sweet plonk (5)
I believe the answer is:
swine
'pig' is the definition.
(I know that pig can be written as swine)
'first sweet plonk' is the wordplay.
'first sweet' becomes 's' (1st letter of 'sweet').
'plonk' becomes 'wine' (I've seen this before).
's'+'wine'='SWINE'
'enjoys' acts as a link.
(Other definitions for swine that I've seen before include "cad" , "Contemptible person; pigs" , "Pig, pigs" , "Archaic pigs, Gadarene in Bible" , "Pig; unpleasant person" .)