Regain heartlessly, but with restraint (4)
I believe the answer is:
rein
'restraint' is the definition.
(I've seen this before)
'regain heartlessly' is the wordplay.
'heartlessly' says to hollow out the word (remove centre letters).
'regain' with its centre taken out is 'REIN'.
'but with' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for rein that I've seen before include "Director's power" , "Part of tack" , "one's attached to a bit" , "given free to experienced riders?" , "Horse's steering strap" .)