Overthrow towards slip (6)
I believe the answer is:
coupon
'slip' is the definition.
(I've seen this before)
'overthrow towards' is the wordplay.
'overthrow' becomes 'coup' (I've seen this in another clue).
'towards' becomes 'on' (I am not sure about this - if you are sure you should believe this answer much more).
'coup'+'on'='COUPON'
Can you help me to learn more?
(Other definitions for coupon that I've seen before include "Token" , "Voucher or ticket" , "Entry form" , "means to buy goods" , "betting slip" .)