One going round hospital in pain (4)
I believe the answer is:
Here is my best explanation:
'pain' is the definition.
(ache is a kind of pain)
'one going round hospital' is the subsidiary indication.
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'going round' means one lot of letters goes inside another.
'hospital' becomes 'h'.
'ace' enclosing 'h' is 'ache'.
'in' is the link.
(Other definitions for ache that I've seen before include "Continuous hurt", "Nagging pain", "A dull, prolonged pain", "Yearn", "Persistent dull pain", "Pain; long (for)", "Twinge", "Longing", "Each (anag.)" and "Similar".)