Companion to back as go between (5)
I believe the answer is:
forth
'between' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?
'companion to back as go' is the wordplay.
I cannot really see how this works, but
'to' could be 'for' and 'for' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for forth that I've seen before include "Edinburgh's river" , "running into view" , "River and estuary in eastern Scotland" , "Onward; the Firth of - -" , "Scottish river and firth" .)