Keen to pay without delay (7)
I believe the answer is:
hotfoot
'without delay' is the definition.
(synonyms)
'keen to pay' is the wordplay.
'keen' becomes 'hot' (hot on something means keen on something).
'to pay' becomes 'foot' (as in footing the bill).
'hot'+'foot'='HOTFOOT'
(Other definitions for hotfoot that I've seen before include "acting quickly?" , "flat out" , "With all possible speed" , "Speedily" , "Experienced by firewalker" .)