Military alliance in collision at once (4)
I believe the answer is:
nato
'military alliance' is the definition.
(I've seen this before)
'in collision at once' is the wordplay.
'in' says the answer is hidden in the clue.
'NATO' can be found hidden inside 'collision at once'.
(Other definitions for nato that I've seen before include "International organisation for collective security" , "Western mil. alliance" , "Collective security alliance engaged in Bosnia" , "Transatlantic alliance" , "Security agreement" .)