Does it matter who's at assembly? (2,4)
I believe the answer is:
so what
'does it matter' is the definition.
(I've seen this before)
'who's at assembly?' is the wordplay.
'assembly?' indicates an anagram (I've seen 'assembly' mean this).
'whos'+'at'='whosat'
'whosat' with letters rearranged gives 'SO WHAT?'.
(Other definitions for so what that I've seen before include "is that surprising?" , "It's unimportant, right?" , "Expresses contempt or lack of interest" , "Does it make any difference" , "I don't care" .)