School volunteers to send aid to arenas (6)
I believe the answer is:
stadia
'to arenas' is the definition.
(plural of 'stadium')
'school volunteers to send aid' is the wordplay.
'school' becomes 'S'.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'to send' is an anagram indicator (I've seen 'sends' mean this (send can mean excite or stir)).
'aid' with letters rearranged gives 'dia'.
's'+'ta'+'dia'='STADIA'
(Other definitions for stadia that I've seen before include "Big venues" , "Athletics grounds" , "large arenas" , "major venues" , "Sports arenas" .)