Facial contortion of severe expert (7)
I believe the answer is:
grimace
'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" .)