lb50 notes held in lawful fashion (5)
I believe the answer is:
legal
'lb50' is the definition.
I can't tell whether this definition defines the answer.
'notes held in lawful fashion' is the wordplay.
'notes' becomes 'ga' (two musical notes).
'held in' is an insertion indicator.
'lawful fashion' becomes 'lel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ga' put inside '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" .)