A severe crack on the face (7)
I believe the answer is:
grimace
'the face' is the definition.
(synonyms)
'a severe crack on' is the wordplay.
'a severe' becomes 'grim' (I can't explain this - if you can you should give a lot more credence to this answer).
'crack' becomes 'ace' (both can informally mean very good).
'on' means one lot of letters go next to another.
'grim'+'ace'='GRIMACE'
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" .)