As well as a number heard (3)
I believe the answer is:
too
'as well as' is the definition.
(I've seen this in another clue)
'a number heard' is the wordplay.
'a number' becomes 'two' (two is a kind of whole number).
'heard' shows a homophone (sound like).
'two' is a homophone of 'TOO'.
(Other definitions for too that I've seen before include "What a pity!" , "As well, also" , "Additionally" , "To a greater than desirable extent" , "On top - also" .)