Crossword Genius

Join forces with again? How could you! (6)

Ross

I believe the answer is:

really

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'could you' is the definition.
I can't judge whether this definition defines the answer.

'join forces with again?' is the wordplay.
'join forces' becomes 'ally' (to ally with someone is to join forces with them).
'with' is a charade indicator (letters next to each other).
'again?' becomes 're' (prefix meaning 'again').
'ally' after 're' is 'REALLY'.

'how' acts as a link.

Can you help me to learn more?

(Other definitions for really that I've seen before include "Go on" , "In genuine way" , "Honestly, it's a fact" , "Actually, no kidding" , "surely not?" .)

I've seen this clue in the Evening Standard.
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