Long service covering hospital (4)
I believe the answer is:
ache
'long' is the definition.
('ache' can be a synonym of 'long')
'service covering hospital' is the wordplay.
'service' becomes 'ace' (ace is a kind of service).
'covering' indicates putting letters inside.
'hospital' becomes 'h'.
'ace' going around 'h' is 'ACHE'.
(Other definitions for ache that I've seen before include "Nagging pain" , "Longing" , "Long ? pain" , "Similar" , "Feel sore" .)