To confine the immature is heartless (6)
I believe the answer is:
immure
'to confine' is the definition.
(immuring is a kind of confining)
'immature is heartless' is the wordplay.
'is heartless' means to remove the middle letters.
'immature' with its middle taken out is 'IMMURE'.
'the' is the link.
(Other definitions for immure that I've seen before include "Imprison, shut in" , "Confine; wall in" , "Sketches" , "Imprison within walls" , "Shut oneself away from social contact" .)