lb50 notes held in lawful fashion (5)
I believe the answer is:
legal
'lb50' is the definition.
I can't tell whether this defines the answer.
'notes held in lawful fashion' is the wordplay.
'notes' becomes 'ga' (two musical notes).
'held in' indicates putting letters inside.
'lawful fashion' becomes 'lel' (I am not sure about this - if you are sure you should believe this answer much more).
'ga' inserted into 'lel' is 'LEGAL'.
Can you help me to learn more?
(Other definitions for legal that I've seen before include "Referring to the law" , "Legitimate" , "Street - (1978 Dylan album)" , "not breaking the law" , "Compliant with the law" .)