Claim a lb50 note at some stage (6)
I believe the answer is:
allege
'claim' is the definition.
(to allege something is to make a claim)
'a lb50 note at some stage' is the wordplay.
'lb50' becomes 'l' (this might be a standard abbreviation which I don't know about).
'note' becomes 'e' (musical note).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'some stage' becomes 'leg' (a leg is a stage of a competition).
'e' put after 'leg' is 'lege'.
'a'+'l'+'lege'='ALLEGE'
Can you help me to learn more?
(Other definitions for allege that I've seen before include "State as a fact without proof" , "Declare or assert without proof" , "Make accusation without proof" , "Aver (without proof)" , "Declare or accuse without proof" .)