Retired group touring Paris, perhaps as a climax (2,3,2,3)
I believe the answer is:
to cap it all
'as a climax' is the definition.
(I've seen this before)
'retired group touring paris perhaps' is the wordplay.
'retired' says the letters should be written in reverse.
'group' becomes 'lot'.
'touring' is an insertion indicator (touring can mean going round).
'paris perhaps' becomes 'capital' (paris is a kind of capital).
'lot' backwards is 'tol'.
'tol' placed around 'capital' is 'TO CAP IT ALL'.
(Other definitions for to cap it all that I've seen before include "the worst of the lot" , "On top of everything else" , "as a last straw" , "Finally" , "To provide the finishing touch" .)