Number in favour announced (4)
I believe the answer is:
four
'number' is the definition.
(I know that four is a deficient number)
'in favour announced' is the wordplay.
'in favour' becomes 'for' (as in being for a particular cause).
'announced' indicates a 'sounds like' (homophone) clue.
'for' is a homophone of 'FOUR'.
(Other definitions for four that I've seen before include "Figure" , "Square root of sixteen" , "Perfect square" , "Number of players in a doubles game" , "Number in quartet" .)