Expert nursing husband in pain (4)
I believe the answer is:
ache
'pain' is the definition.
(ache is a kind of pain)
'expert nursing husband' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'nursing' is an insertion indicator.
'husband' becomes 'h'.
'ace' enclosing 'h' is 'ACHE'.
'in' is the link.
(Other definitions for ache that I've seen before include "Dull prolonged pain" , "Persistent dull pain" , "Pine" , "Hurt; yearn" , "Shakespearean character" .)