Idiomatic expression initially spelled out differently, in other words (4,2,2,3)
I believe the answer is:
that is to say
'in other words' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for that is to say that I've seen before include "in other words" , "Let me explain further" , "I should add" , "Ie" .)