Duke holding base adult party (6)
I believe the answer is:
fiesta
'party' is the definition.
(thesaurus)
'duke holding base adult' is the wordplay.
'duke' becomes 'fist' (put up your dukes).
'holding' is an insertion indicator.
'base' becomes 'e' (I've seen this in other clues**).
'adult' becomes 'a'.
'fist' enclosing 'e' is 'fiest'.
'fiest'+'a'='FIESTA'
(Other definitions for fiesta that I've seen before include "Religious festival in Hispanic communities" , "foreign holiday" , "public event abroad" , "Outdoor celebration" , "Holiday; carnival" .)