Representative is mature, not heartless (5)
I believe the answer is:
agent
'representative' is the definition.
(agent is a kind of representative)
'mature not heartless' is the wordplay.
'mature' becomes 'age' (synonyms).
'heartless' means to remove the middle letters.
'not' with its centre taken out is 'nt'.
'age'+'nt'='AGENT'
'is' acts as a link.
(Other definitions for agent that I've seen before include "Substance that exerts some force or effect" , "Vehicle" , "Factor, spy" , "Double-zero-seven, for example" , "Salesman" .)