What's more, many back taking on Brussels, for example (2,3,2,3)
I believe the answer is:
to cap it all
'what's more' is the definition.
(both mean 'on top of everything else...')
'many back taking on brussels for example' is the wordplay.
'many' becomes 'lot' (a lot of something).
'back' says the letters should be written backwards.
'taking on' means one lot of letters goes inside another.
'brussels for example' becomes 'capital' (brussels is a kind of capital).
'lot' backwards is 'tol'.
'tol' enclosing 'capital' is 'TO CAP IT ALL'.
(Other definitions for to cap it all that I've seen before include "As a (frequently unwanted) climax" , "To provide the finishing touch" , "the worst of the lot" , "Finally" , "On top of everything else" .)