Fellow in hospital wearing hat (4)
I believe the answer is:
chap
'fellow' is the definition.
('chap' can be a synonym of 'fellow')
'hospital wearing hat' is the wordplay.
'hospital' becomes 'h'.
'wearing' indicates putting letters inside.
'hat' becomes 'cap' (I've seen this before).
'h' put inside 'cap' is 'CHAP'.
'in' is the link.
(Other definitions for chap that I've seen before include "Man; (of skin) turn sore" , "Man, sore patch" , "Skin crack due to cold - bloke" , "Fish" , "Man, boy, fellow" .)