Sees man collapsing all at once (2,5)
I believe the answer is:
en masse
'all at once' is the definition.
(I've seen this in another clue)
'sees man collapsing' is the wordplay.
'collapsing' is an anagram indicator.
'sees'+'man'='seesman'
'seesman' with letters rearranged gives 'EN MASSE'.
(Other definitions for en masse that I've seen before include "All together - in France (2.5)" , "Sees man (anag.)" , "in concert" , "All together as a group" , "As a body (Fr.)" .)