Heartless relative remains free (7)
I believe the answer is:
unleash
'free' is the definition.
(I've seen this in another clue)
'heartless relative remains' is the wordplay.
'heartless' suggests removing the centre.
'relative' becomes 'uncle' (uncle is a kind of relative).
'remains' becomes 'ash' (remains of a fire).
'uncle' with its centre removed is 'unle'.
'unle'+'ash'='UNLEASH'
(Other definitions for unleash that I've seen before include "Let off the lead" , "Free from control" , "Set free from restraint, say a dog" , "Release from restraint" , "Flee from restraint, allow to pursue like dog" .)