One outside hospital in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'one outside hospital' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'outside' is an insertion indicator.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'ace' placed around 'h' is 'ACHE'.
'in' is the link.
(Other definitions for ache that I've seen before include "Persistent dull pain" , "Dull continuous pain" , "Long ? pain" , "Dull throbbing pain" , "suffering" .)