Mates changing sides (5)
I believe the answer is:
teams
'sides' is the definition.
(I've seen this in another clue)
'mates changing' is the wordplay.
'changing' indicates an anagram.
'mates' with letters rearranged gives 'TEAMS'.
(Other definitions for teams that I've seen before include "Lots of players" , "Crews" , "Groups or troupes" , "Groups of people working together" , "Sports sides" .)