Agent’s letter from abroad left near (6)
I believe the answer is:
philby
'agent's' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
'letter from abroad left near' is the wordplay.
'letter from abroad' becomes 'phi' (letter of the Greek alphabet).
'left' becomes 'l' (common abbreviation).
'near' becomes 'by' (near means close by).
'phi'+'l'+'by'='PHILBY'
Can you help me to learn more?
(Other definitions for philby that I've seen before include "British agent associated with Burgess, MacLean" , "Kim --, notorious spy" .)