Page is one trained for secret work (9)
I believe the answer is:
espionage
'secret work' is the definition.
(I've seen this before)
'page is one trained' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'page'+'is'+'one'='pageisone'
'pageisone' with letters rearranged gives 'ESPIONAGE'.
'for' acts as a link.
(Other definitions for espionage that I've seen before include "agent's job" , "intelligence" , "hush-hush work" , "seeking information" , "Practice of spying" .)