They say, what perfumer does, is perfectly reasonable (5,5)

'they say what perfumer' is the definition.
The definition suggests an adverb but the answer is not.

'does is perfectly reasonable' is the wordplay.
I cannot really understand how this works, but
'does' could be 'makes' ('make' can be a synonym of 'do') and 'makes' is located in the answer.
'reasonable' could be 'sense' (I've seen this before) and 'sense' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

Can you help me to learn more?