Restrain a broken hearted friend? (4)
I believe the answer is:
rein
'restrain' is the definition.
(reining is a kind of restraining)
'a broken hearted friend?' is the wordplay.
'a broken' is an anagram indicator.
'hearted' means to look at the middle letters.
The central letters of 'friend' are 'rien'.
'rien' is an anagram of 'REIN'.
(Other definitions for rein that I've seen before include "Part of tack" , "leading article?" , "Strap to control horse" , "given free to experienced riders?" , "Long strap to restrain" .)