Preposition used in pools coupons (4)
I believe the answer is:
'preposition' is the definition.
(I've seen this before)
'in pools coupons' is the wordplay.
'in' says the answer is hidden in the clue.
'UPON' can be found hidden inside 'pools coupons'.
'used' is the link.
(Other definitions for upon that I've seen before include "Atop" , "Abreast of, au fait with" , "Thereafter" , "Immediately after" , "(Placed) on top of" .)