Zany crew gatecrashing start of school dance (9)
I believe the answer is:
screwball
'zany' is the definition.
(I've seen this before)
'crew gatecrashing start of school dance' is the wordplay.
'gatecrashing' indicates putting letters inside.
'start of' says to take the initial letters.
'dance' becomes 'ball' (ball is a kind of dance).
The initial letter of 'school' is 's'.
's'+'ball'='sball'
'crew' going inside 'sball' is 'SCREWBALL'.
(Other definitions for screwball that I've seen before include "Crazy (person)" , "Crazy American" , "touched person" , "Card" , "Eccentric" .)