Spy is a decent chap (5)
I believe the answer is:
agent
'spy' is the definition.
(I've seen this before)
'a decent chap' is the wordplay.
'decent chap' becomes 'gent' (gent is short for gentleman).
'a'+'gent'='AGENT'
'is' is the link.
(Other definitions for agent that I've seen before include "One acting on behalf of another" , "Double-zero-seven, for example" , "Sales rep" , "Salesman" , "Delegate" .)