Maturity, not without heart, is a factor (5)
I believe the answer is:
agent
'factor' is the definition.
(I've seen this before)
'maturity not without heart' is the wordplay.
'maturity' becomes 'age' (I've seen this before).
'without heart' suggests removing the centre.
'not' with its middle removed is 'nt'.
'age'+'nt'='AGENT'
'is a' acts as a link.
(Other definitions for agent that I've seen before include "One representing others" , "Representative - means" , "Traveller" , "Double-zero-seven, for example" , "Operative" .)