They're literally chums (3,4)
I believe the answer is:
pen pals
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'literally chums' is the wordplay.
'literally' becomes 'pen' (I can't justify this - if you can you should believe this answer much more).
'chums' becomes 'pals' ('pal' can be a synonym of 'chum').
'pen'+'pals'='PEN PALS'
Can you help me to learn more?
(Other definitions for pen pals that I've seen before include "'Friends through correspondence (3,4)'" , "Friends who communicate in writing" , "Young correspondents" , "They correspond" .)