Age ponies developed intelligence (9)
I believe the answer is:
espionage
'intelligence' is the definition.
(espionage is a kind of intelligence)
'age ponies developed' is the wordplay.
'developed' is an anagram indicator.
'age'+'ponies'='ageponies'
'ageponies' with letters rearranged gives 'ESPIONAGE'.
(Other definitions for espionage that I've seen before include "007's game" , "agent's job" , "Practice of spying" , "undercover work" , "hush-hush work" .)