Playing mostly rock, host support bands (7)

'bands' is the definition.
(I know this)

'playing mostly rock host support' is the wordplay.
'playing' indicates anagramming the letters.
'mostly' means to remove the last letter (most but not all of the word).
'support' says to put letters next to each other (in a down clue, some letters hold others up).
'rock' with its final letter taken away is 'roc'.
'roc'+'host'='rochost'
'rochost' with letters rearranged gives 'COHORTS'.