Prepare a formal note (5)
I believe the answer is:
prime
'prepare' is the definition.
(to prime is to prepare or make ready)
'a formal note' is the wordplay.
'a formal' becomes 'prim' (I've seen this before).
'note' becomes 'e' (musical note).
'prim'+'e'='PRIME'
(Other definitions for prime that I've seen before include "Original" , "Number with no factor but itself and 1" , "Maybe eleven" , "Foremost" , "golden age?" .)