Parisian is by mountains in Australia in colourful outer layer (6,4)

'colourful outer layer' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'parisian is by mountains in australia' is the wordplay.
'parisian is' becomes 'est' ('is' in French).
'by' is a charade indicator (letters next to each other).
'mountains' becomes 'range' (range of mountains).
'in' means one lot of letters goes inside another.
'australia' becomes 'oz' (informal name for Australia).
'range' put within 'oz' is 'orangez'.
'est' after 'orangez' is 'ORANGE ZEST'.

'in' acts as a link.

Can you help me to learn more?