Fight most heartlessly with husband for affection (6)
I believe the answer is:
warmth
'affection' is the definition.
(I've seen this before)
'fight most heartlessly with husband' is the wordplay.
'fight' becomes 'war' (warring is a kind of fighting).
'heartlessly' suggests removing the centre.
'with' means one lot of letters go next to another.
'husband' becomes 'h' (abbreviation).
'most' with its middle removed is 'mt'.
'war'+'mt'+'h'='WARMTH'
'for' is the link.
(Other definitions for warmth that I've seen before include "Cordiality" , "Intensity of emotion" , "tenderness" , "Feeling" , "Pleasant heat" .)