A pigeon's trained by English for spying (9)
I believe the answer is:
espionage
'for spying' is the definition.
(espionage is a kind of spying)
'a pigeon's trained by english' is the wordplay.
'trained' indicates an anagram (train can mean to shape or manipulate).
'by' means one lot of letters go next to another.
'english' becomes 'E' (abbreviation).
'a'+'pigeons'='apigeons'
'apigeons' with letters rearranged gives 'espionag'.
'espionag'+'e'='ESPIONAGE'
(Other definitions for espionage that I've seen before include "secret work" , "hush-hush work" , "seeking information" , "Practice of spying" , "undercover work" .)