Ace catch includes new spy (5)
I believe the answer is:
agent
'spy' is the definition.
(I've seen this in another clue)
'ace catch includes new' is the wordplay.
'ace' becomes 'a'.
'catch' becomes 'get' (synonyms).
'includes' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'get' placed around 'n' is 'gent'.
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "Spy; service provider" , "Performer's manager" , "One may be working" , "Instrument" , "who can act?" .)