Half the ransom note is obviously not all (4)
I believe the answer is:
some
'not all' is the definition.
('some' is the opposite to 'all')
'half the ransom note' is the wordplay.
'half' means to take half the letters.
'note' becomes 'e' (musical note**).
'ransom' halved is 'som'.
'som'+'e'='SOME'
'is obviously' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for some that I've seen before include "Hard work" , "certain" , "Several" , "An unspecified quantity, not all" , "A little; not a little" .)