Pain expert nursing husband (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 can informally mean very good).
'nursing' means one lot of letters goes inside another.
'husband' becomes 'h'.
'ace' going around 'h' is 'ACHE'.
(Other definitions for ache that I've seen before include "suffering" , "Twinge" , "strong feeling" , "Longing" , "Nagging pain" .)