Pain expert outside hospital (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'expert outside hospital' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'outside' indicates putting letters inside.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'ace' placed around 'h' is 'ACHE'.
(Other definitions for ache that I've seen before include "yearning" , "Twinge" , "suffering" , "Dull continuous pain" , "Similar" .)