Chlorine always bungs up brainbox (6,5)
I believe the answer is:
clever clogs
'up brainbox' is the definition.
I know nothing about this answer so I can't tell whether this works.
'chlorine always bungs' is the wordplay.
'chlorine' becomes 'Cl' (Cl is the chemical symbol for chlorine).
'always' becomes 'ever' (synonyms).
'bungs' becomes 'clogs' (I can't explain this - if you can you should believe this answer much more).
'cl'+'ever'+'clogs'='CLEVER CLOGS'
Can you help me to learn more?
(Other definitions for clever clogs that I've seen before include "Smarties" , "Know-all" , "Smarty-boots?" , "Smartypants" , "Wise guy" .)