Allows that one is in pain (7)
I believe the answer is:
suffers
This clue could be a double definition.
'allows' is the first definition.
(suffering is a kind of allowing)
'is in pain' is the second definition.
('suffer' can be a synonym of 'be in pain')
'that one' acts as a link.
This may not be correct. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for suffers that I've seen before include "Abides" , "Undergoes" .)