On top, so to speak (3)
I believe the answer is:
too
'on top' is the definition.
(on top can mean in addition or as well)
'to speak' is the wordplay.
'speak' shows a homophone (sound like).
'to' is a homophone of 'TOO'.
'so' is the link.
(Other definitions for too that I've seen before include "Additionally" , "To a greater than desirable extent" , "Moreover, in addition" , "What a pity!" , "On top - also" .)