Care of middle school cards (9)
I believe the answer is:
comedians
'cards' is the definition.
(I've seen this in another clue)
'care of middle school' is the wordplay.
'care of' becomes 'co' (c/o can mean 'care of' when addressing mail).
'middle' becomes 'median' (I've seen this in another clue).
'school' becomes 'S'.
'co'+'median'+'s'='COMEDIANS'
(Other definitions for comedians that I've seen before include "MB etc" , "Entertainers" .)