Master almost changed sides (5)
I believe the answer is:
teams
'sides' is the definition.
(I've seen this in another clue)
'master almost changed' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'changed' indicates an anagram.
'master' with its final letter taken off is 'maste'.
'maste' with letters rearranged gives 'TEAMS'.
(Other definitions for teams that I've seen before include "Squads" , "Crews" , "Groups of people working together" , "Lots of players" , "Groups or troupes" .)