Join forces with again? How could you! (6)
I believe the answer is:
really
'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?" .)