Make invalid filling of lush doughnut, say (7)
I believe the answer is:
annulus
'doughnut say' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'make invalid filling of lush' is the wordplay.
'make invalid' becomes 'annul' (I've seen this before).
'filling of' means to look at the middle letters (the letters which fill the inside of the word).
The centre of 'lush' is 'us'.
'annul'+'us'='ANNULUS'
Can you help me to learn more?
(Other definitions for annulus that I've seen before include "Ring-shaped object" , "Ring-shaped structure" .)