One observing a Serb spy in disguise (6-2)
I believe the answer is:
passer-by
'one' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association between them that I can't see?
'a serb spy in disguise' is the wordplay.
'in disguise' indicates an anagram.
'a'+'serb'+'spy'='aserbspy'
'aserbspy' is an anagram of 'PASSER-BY'.
'observing' is the link.
Can you help me to learn more?
(Other definitions for passer-by that I've seen before include "Spectator not stopping?" , "Witness" , "Someone coming near" , "One walking near" , "One happening past" .)