Owing small fees (4)
I believe the answer is:
dues
'fees' is the definition.
(payments which are due)
'owing small' is the wordplay.
'owing' becomes 'due' (similar in meaning).
'small' becomes 's' (abbreviation - e.g. clothes size).
'due'+'s'='DUES'
(Other definitions for dues that I've seen before include "Membership fees" , "Payments made for upkeep of church" , "Subscription to a club or society" , "subs" .)