Pain in hospital accepted by expert (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'hospital accepted by expert' is the wordplay.
'hospital' becomes 'h'.
'accepted by' indicates putting letters inside.
'expert' becomes 'ace' (ace is a kind of expert).
'h' put inside 'ace' is 'ACHE'.
'in' is the link.
(Other definitions for ache that I've seen before include "Long ? pain" , "Continuous hurt" , "Feel pain" , "Play up" , "Shakespearean character" .)