Criterion for a jester (10)
I believe the answer is:
touchstone
I believe this is a double definition.
'criterion' is the first definition.
(I know that criterion can be written as touchstone)
'jester' is the second definition.
(I've seen this before)
'for a' acts as a link.
(Other definitions for touchstone that I've seen before include "stage clown" , "Character in As You Like It" , "Standard" , "jester" , "Shakespearean character" .)