Number, so we hear, in front (4)
I believe the answer is:
fore
'front' is the definition.
(I know that fore is a type of front)
'number so we hear' is the wordplay.
'number' becomes 'four' (four is a kind of number).
'so we hear' shows a homophone (sound like).
'four' is a homophone of 'FORE'.
'in' is the link.
(Other definitions for fore that I've seen before include "Warning cry of golfer" , "Ancestor" , "Not aft" , "Placed in front" , "The front part of a ship" .)