Initially angina causes horrible excruciating pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'initially angina causes horrible excruciating' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'angina causes horrible excruciating' is 'ACHE'.
(Other definitions for ache that I've seen before include "Long ? pain" , "painful" , "Cause pain" , "burn" , "Hanker" .)