Hadn't paid for the round we had (4)
I believe the answer is:
owed
'hadn't paid' is the definition.
(I've seen this in another clue)
'round we had' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'we had' becomes 'we'd' (contraction of 'we had').
'o'+'wed'='OWED'
'for the' acts as a link.
(Other definitions for owed that I've seen before include "Was due to pay" , "Was indebted" , "Outstanding" , "Was obliged to pay" , "(Sum) due for payment" .)