Put up with regular heartlessness? (4)
I believe the answer is:
rear
'put up' is the definition.
(I know that put up can be written as rear)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for rear that I've seen before include "The back part of something" , "what you may bring up" , "Bring up or stand up on hind legs" , "Jump" , "Of horse, rise up on hind legs" .)