Scattered SAS men finally strike as one (2,5)
I believe the answer is:
en masse
'as one' is the definition.
(both can mean as a group)
'scattered sas men finally strike' is the wordplay.
'scattered' indicates an anagram.
'finally' indicates one should take the final letters.
The final letter of 'strike' is 'e'.
'sas'+'men'='sasmen'
'sasmen' with letters rearranged gives 'enmass'.
'enmass'+'e'='EN MASSE'
(Other definitions for en masse that I've seen before include "All together - in France (2.5)" , "as a bunch" , "All together (Fr.)" , "Sees man (anag.)" , "In force" .)