Not everyone stands up in a church together (3,2,4)
I believe the answer is:
all at once
'together' is the definition.
(I've seen this before)
'not everyone stands up in a church' is the wordplay.
'everyone' becomes 'all' (all people).
'stands up' shows that the letters should be reversed in order (in a down clue, letters go up).
'in' indicates putting letters inside.
'church' becomes 'CE' (abbreviation for Church of England).
'not'+'all'='notall'
'notall' backwards is 'llaton'.
'a'+'ce'='ace'
'llaton' placed into 'ace' is 'ALL AT ONCE'.
(Other definitions for all at once that I've seen before include "Without warning" , "Simultaneously - immediately" , "Suddenly" , "Together" , "How daffodils appeared to Wordsworth" .)