After review Serb pays possible witness (6-2)
I believe the answer is:
passer-by
'possible witness' is the definition.
(a passer-by might have witnessed an event)
'after review serb pays' is the wordplay.
'after review' indicates an anagram (letters reviewed or revised).
'serb'+'pays'='serbpays'
'serbpays' anagrammed gives 'PASSER-BY'.
(Other definitions for passer-by that I've seen before include "One walking near" , "One happening past" , "Person walking past" , "Person who happens to be going past" , "Someone coming near" .)