Essentially boasting on wine (4)
I believe the answer is:
asti
'wine' is the definition.
(I've seen this before)
'essentially boasting on' is the wordplay.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'ASTI' is hidden in the letters of 'boasting on'.
(Other definitions for asti that I've seen before include "It's a (anag) - wine" , "Italian drunk" , "Bubbly wine" , "Capital of a north-west Italian province, known for its sparking wine" , "town in Italy" .)