Crossword Genius

Facial contortion of severe expert (7)

Ross

I believe the answer is:

grimace

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'facial contortion' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'severe expert' is the wordplay.
'severe' becomes 'grim' (I can't explain this - if you can you should give a lot more credence to this answer).
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for grimace that I've seen before include "Wry look" , "Distorted facial expression" , "Glower" , "(Make) a wry face" , "Pull a face" .)

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