In favour of a number, say (3)
I believe the answer is:
for
'in favour of' is the definition.
(being for a particular cause)
'a number say' is the wordplay.
'a number' becomes 'four' (four is a kind of number).
'say' indicates a 'sounds like' (homophone) clue.
'four' sounds like 'FOR'.
(Other definitions for for that I've seen before include "On behalf of" , "Favouring" , "In support of" , "See 24" , "Pro" .)