Spy's a decent chap (5)
I believe the answer is:
agent
'spy's' is the definition.
(thesaurus)
'a decent chap' is the wordplay.
'decent chap' becomes 'gent' (I've seen this in other clues. I am not sure about the 'decent' bit.**).
'a'+'gent'='AGENT'
Can you help me to learn more?
(Other definitions for agent that I've seen before include "Bond, e.g." , "Factor, spy" , "who can act?" , "One does business on behalf of another" , "Representative - means" .)