Token overthrow of rulers feasible (6)
I believe the answer is:
coupon
'token' is the definition.
(I've seen this before)
'overthrow of rulers feasible' is the wordplay.
'overthrow of rulers' becomes 'coup' (as in a coup d'etat).
'feasible' becomes 'on' (as opposed to 'not on' -unacceptable or unfeasible).
'coup'+'on'='COUPON'
(Other definitions for coupon that I've seen before include "Token" , "Voucher or ticket" , "betting slip" , "Entry form" , "means to buy goods" .)