A well-known name in socks (5)
I believe the answer is:
'a well-known' is the definition.
I can't judge whether this definition defines the answer.
'name in socks' is the wordplay.
'name' becomes 'Bob' (short for Robert).
'in socks' becomes 'by' (I am not sure about this - if you are sure you should believe this answer much more).
Can you help me to learn more?
(Other definitions for bobby that I've seen before include "- Moore, World Cup winning captain" , "A -dazzler, a -soxer" , "-- Moore; -- Charlton (footballers)" , "Cop (slang)" , "Policeman (colloq.)" .)