Preposition used in pools coupons (4)
I believe the answer is:
upon
'preposition' is the definition.
(I've seen this before)
'in pools coupons' is the wordplay.
'in' indicates the answer is hidden within 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" .)