Meet the cost of accommodating second Balkan potential witness (6-2)
I believe the answer is:
passer-by
'potential witness' is the definition.
'meet the cost of accommodating second balkan' is the wordplay.
'meet the cost of' becomes 'pay'.
'accommodating' indicates putting letters inside.
'second' becomes 's' (common abbreviation).
'balkan' becomes 'serb'.
's'+'serb'='sserb'
'pay' placed around 'sserb' is 'PASSER-BY'.
(Other definitions for passer-by that I've seen before include "One happening past" , "Someone coming near" , "Person who happens to be going past" , "One walking near" , "Witness" .)