Suffer mental anguish in hospital visiting one (4)
I believe the answer is:
ache
'suffer mental anguish' is the definition.
(as in aching or pining for someone)
'hospital visiting one' is the wordplay.
'hospital' becomes 'H' (abbreviation for hospital on maps).
'visiting' means one lot of letters goes inside another.
'one' becomes 'ace' (ace has a value of one in some card games).
'h' going inside 'ace' is 'ACHE'.
'in' acts as a link.
(Other definitions for ache that I've seen before include "Twinge" , "strong feeling" , "Play up" , "Hurt; yearn" , "Take an aspirin for it" .)