A companion beginning to exaggerate pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'a companion beginning to exaggerate' is the wordplay.
'companion' becomes 'ch' (I've seen this in other clues).
'beginning to' suggests taking the first letters.
The first letter of 'exaggerate' is 'e'.
'a'+'ch'+'e'='ACHE'
(Other definitions for ache that I've seen before include "Pine" , "Similar" , "Take an aspirin for it" , "Shakespearean character" , "agony" .)