Spy taking time, not losing heart (5)
I believe the answer is:
agent
'spy' is the definition.
(thesaurus)
'time not losing heart' is the wordplay.
'time' becomes 'age' (age is a kind of time**).
'losing heart' means to remove the middle letters.
'not' with its centre taken out is 'nt'.
'age'+'nt'='AGENT'
'taking' acts as a link.
(Other definitions for agent that I've seen before include "Substance that exerts some force or effect" , "who can act?" , "Business manager" , "Person acting on behalf of another" , "Salesman" .)