Serious cut close to ankle (6)
I believe the answer is:
'serious' is the definition.
('severe' can be a synonym of 'serious')
'cut close to ankle' is the wordplay.
'cut' becomes 'sever' (severing is a kind of cutting).
'close to' indicates one should take the final letters (the close/ending of).
The final letter of 'ankle' is 'e'.
(Other definitions for severe that I've seen before include "Strict, hard", "Draconian", "Harsh and punitive", "Formal - unsmiling", "Harsh; very plain".)