Change man sees in a group (2,5)
I believe the answer is:
en masse
'in a group' is the definition.
(I've seen this in another clue)
'change man sees' is the wordplay.
'change' is an anagram indicator.
'man'+'sees'='mansees'
'mansees' anagrammed gives 'EN MASSE'.
(Other definitions for en masse that I've seen before include "In force" , "Many as one" , "In a group U altogether" , "All together as a group" , "All together - in France (2.5)" .)