Bobby's letter to the aforesaid robber (6)
I believe the answer is:
robert
'aforesaid robber' is the definition.
The definition suggests a singular noun which matches the answer.
'bobby's letter to the' is the wordplay.
'bobby's' becomes 'robe' (I can't explain this - if you can you should believe this answer much more).
'letter' becomes 'r'.
'to the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'robe'+'r'+'t'='ROBERT'
Can you help me to learn more?
(Other definitions for robert that I've seen before include "Founder of London's police force" , "-- the Bruce" , "- Browning, Eng. poet" , "See 22" , "- Edward Lee, US Confederate general" .)