Husband, not totally passionate, to become less sympathetic (6)
I believe the answer is:
harden
'become less sympathetic' is the definition.
(I've seen this before)
'husband not totally passionate' is the wordplay.
'husband' becomes 'h'.
'not totally' means to remove the last letter.
'passionate' becomes 'ardent' ('ardent' can be similar in meaning to 'passionate').
'ardent' with its final letter taken off is 'arden'.
'h'+'arden'='HARDEN'
'to' is the link.
(Other definitions for harden that I've seen before include "Set; become tougher" , "Become firm or tough" , "Steel oneself" , "Small fly" , "Temper, steel say" .)