One observing a Serb spy in disguise (6-2)

I believe the answer is:

passer-by

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'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" .)

I've seen this clue in The Times.

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