Mad bishop confronting host (5)
I believe the answer is:
barmy
'mad' is the definition.
(informal term)
'bishop confronting host' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'confronting' is a charade indicator (letters next to each other).
'host' becomes 'army' (both can mean a large number of people).
'b'+'army'='BARMY'
(Other definitions for barmy that I've seen before include "Crackers, daft" , "Bonkers" , "Extremely foolish" , "Mad (like footballing "army")" , "like Basil, arguably?" .)