A severe crack on the face (7)

I believe the answer is:

grimace

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'the face' is the definition.
(synonyms)

'a severe crack' is the wordplay.
'a severe' becomes 'grim' (I am not sure about this - if you are sure you should believe this answer much more).
'crack' becomes 'ace' (I've seen this before).
'grim'+'ace'='GRIMACE'

'on' acts as a link.

Can you help me to learn more?

(Other definitions for grimace that I've seen before include "Wry look" , "Contort the face" , "Nasty expression" , "Ugly facial expression" , "Facial contortion expressive of pain or disgust" .)

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