One nursing husband in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'one nursing husband' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'nursing' indicates putting letters inside.
'husband' becomes 'h' (abbreviation).
'ace' placed around 'h' is 'ACHE'.
'in' is the link.
(Other definitions for ache that I've seen before include "painful" , "Longing" , "Suffer dull pain" , "suffering" , "Similar" .)