Didn't side with anybody - didn't join in at all (4,2,4)

'didn't' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'side with anybody didn't join in at all' is the wordplay.
I cannot quite understand how this works, but
'side' could be 'r' (abbreviation for right) and 'r' is found in the answer.
'at' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?