Opera on banks of Loire is in proportion (2,5)
I believe the answer is:
to scale
'in proportion' is the definition.
(I've seen this before)
'opera on banks of loire' is the wordplay.
'opera' becomes 'tosca' (Giacomo Puccini opera).
'on' says to put letters next to each other.
'banks of' says to hollow out the word (remove centre letters) (the banks are the sides of the word).
'loire' with its middle taken out is 'le'.
'tosca'+'le'='TO SCALE'
'is' is the link.
(Other definitions for to scale that I've seen before include "relatively accurate" , "properly modelled" , "The best models may be" , "produce with uniform reduction" , "Matching original" .)