Crossword Genius

They're literally chums (3,4)

Ross

I believe the answer is:

pen pals

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'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" .)

I've seen this clue in the Evening Standard.
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