Judge fair queen to be fool (5)
I believe the answer is:
joker
'be fool' is the definition.
(thesaurus)
'judge fair queen' is the wordplay.
'judge' becomes 'J' (abbreviation for judge).
'fair' becomes 'ok'.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'j'+'ok'+'er'='JOKER'
'to' acts as a link.
(Other definitions for joker that I've seen before include "Card - wag" , "Comic; a card" , "Humorist" , "Comedian" , "trickster" .)