Fool jolly officer (4)
I believe the answer is:
dolt
'fool' is the definition.
(thesaurus)
'jolly officer' is the wordplay.
'jolly' becomes 'do' (both can mean a party).
'officer' becomes 'lt' (I've seen this in other clues).
'do'+'lt'='DOLT'
(Other definitions for dolt that I've seen before include "Dull or stupid fellow" , "Dunce" , "dope" , "Blockhead" , "Slow-witted person" .)