Heartless couple joining comedian, that is to say (2,3)
I believe the answer is:
to wit
'that is to say' is the definition.
(I've seen this before)
'heartless couple joining comedian' is the wordplay.
'heartless' suggests removing the centre.
'couple' becomes 'two' (two people or things).
'joining' is a charade indicator (letters next to each other).
'comedian' becomes 'wit' (a witty person).
'two' with its centre removed is 'to'.
'to'+'wit'='TO WIT'
(Other definitions for to wit that I've seen before include "Namely" , "i.e." , "specifically" , "In other words" , "what to do with a broken-down car!" .)