Sides with, or switches before saying "cheerio" (6,2)
I believe the answer is:
sticks by
'sides with' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their -s form.
Maybe you can see a link between them that I can't see?
'switches before saying cheerio' is the wordplay.
'switches before' becomes 'sticks' (I am not sure about this - if you are sure you should believe this answer much more).
'saying' shows a homophone (sound like).
'cheerio' becomes 'bye' (synonyms).
'bye' is a homophone of 'by'.
'sticks'+'by'='STICKS BY'
'or' is the link.
Can you help me to learn more?
(Another definition for sticks by that I've seen is " Remains loyal".)