Number in favour of being heard (4)
I believe the answer is:
four
'number' is the definition.
(I know that four is a deficient number)
'in favour of being heard' is the wordplay.
'in favour of' becomes 'for' (being for a particular cause).
'being heard' shows a homophone (sound like).
'for' sounds like 'FOUR'.
(Other definitions for four that I've seen before include "Two squared" , "Set of bridge players?" , "Cardinal" , "Square root of sixteen" , "Square number" .)