Fools to judge, mostly (5)
I believe the answer is:
asses
'fools' is the definition.
(ass can mean a foolish person)
'judge mostly' is the wordplay.
'judge' becomes 'assess' (to assess is to judge or rate).
'mostly' means to remove the last letter (most but not all of the word).
'assess' with its last letter taken off is 'ASSES'.
'to' is the link.
(Other definitions for asses that I've seen before include "not the most sensible types" , "Fools; animals" , "Pompous fools" , "More than one fool" , "dim bulbs?" .)