Expert son training American spies on record (10)
I believe the answer is:
specialist
'expert' is the definition.
(I know that specialist is a type of expert)
'son training american spies on record' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'training' becomes 'PE' (short for Physical Education).
'american spies' becomes 'CIA' (Central Intelligence Agency).
'on' means one lot of letters go next to another.
'record' becomes 'list' (list is a type of record).
's'+'pe'+'cia'+'list'='SPECIALIST'
(Other definitions for specialist that I've seen before include "One skilled in a limited field" , "One skilled in a specific field" , "Expert" , "particular skill" .)