Runs only slowly, having a bad heart (5)
I believe the answer is:
trots
'runs' is the definition.
(trot is a kind of run)
'only slowly having a bad heart' is the wordplay.
I cannot quite see how this works, but
'bad' could be 'rot' (I've seen this before) and 'rot' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for trots that I've seen before include "Runs slowly" , "jogs" .)