One nursing husband is to be tender (4)
I believe the answer is:
ache
'be tender' is the definition.
(a body part that aches might be tender)
'one nursing husband' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'nursing' is an insertion indicator.
'husband' becomes 'h' (abbreviation).
'ace' enclosing 'h' is 'ACHE'.
'is to' acts as a link.
(Other definitions for ache that I've seen before include "Dull prolonged pain" , "Each (anag.)" , "Hanker" , "Take an aspirin for it" , "Play up" .)