Using humour on husband (4)
I believe the answer is:
with
'using' is the definition.
(eg 'fixed with glue' means 'fixed using glue')
'humour on husband' is the wordplay.
'humour' becomes 'wit' (synonyms).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'husband' becomes 'h' (abbreviation).
'wit'+'h'='WITH'
(Other definitions for with that I've seen before include "In spite of" , "By means of" , "See 5" , "Alongside" , "understanding" .)