Guinness, but first sting the knowall (5,4)
I believe the answer is:
smart alec
'guinness' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'first sting the knowall' is the wordplay.
'first' means one lot of letters go next to another.
'sting' becomes 'smart' (I've seen this before).
'the knowall' becomes 'alec' (I can't explain this - if you can you should believe this answer much more).
'smart'+'alec' is 'SMART-ALEC'.
'but' is the link.
Can you help me to learn more?
(Other definitions for smart alec that I've seen before include "Clever person" , "wise guy" , "Know-all" , "Clever Dick" , "Clever sort" .)