The setter's model is exact (6)
I believe the answer is:
impose
'exact' is the definition.
(thesaurus)
'the setter's model' is the wordplay.
'the setter's' becomes 'im' (I've seen this in other clues).
'model' becomes 'pose' ('pose' can be a synonym of 'model').
'im'+'pose'='IMPOSE'
'is' acts as a link.
(Other definitions for impose that I've seen before include "Put burden or tax on" , "Make demands of" , "'Levy, inflict (6)'" , "Put on, tax or obligation" , "Decree" .)