Receives key to set arrangement (4)
I believe the answer is:
gets
'receives' is the definition.
('get' can be a synonym of 'receive')
'key to set arrangement' is the wordplay.
'key' becomes 'g' (musical key).
'to' is a charade indicator (letters next to each other).
'arrangement' is an anagram indicator.
'set' with letters rearranged gives 'ets'.
'g'+'ets'='GETS'
(Other definitions for gets that I've seen before include "Fetches or receives" , "Acquires, obtains" , "Obtains or becomes" , "Understands" , "Notices" .)