Everybody attending available to converge outside in a rush (3,2,4)
I believe the answer is:
all at once
'in a rush' is the definition.
I can't tell whether this defines the answer.
'everybody attending available to converge outside' is the wordplay.
'everybody' becomes 'all' (all people).
'attending' becomes 'at' (e.g. at school means attending school).
'available' becomes 'on' (on can mean activated or available).
'to' is a charade indicator (letters next to each other).
'outside' means to remove the middle letters.
'converge' with its centre removed is 'ce'.
'all'+'at'+'on'+'ce'='ALL AT ONCE'
Can you help me to learn more?
(Other definitions for all at once that I've seen before include "Together" , "How daffodils appeared to Wordsworth" , "in unison" , "Simultaneously - immediately" , "Suddenly" .)