Harsh to lop off earl's head (6)
I believe the answer is:
severe
'harsh' is the definition.
(I've seen this before)
'lop off earl's head' is the wordplay.
'lop off' becomes 'sever' (I've seen this before).
'head' says to take the initial letters.
The first letter of 'earls' is 'e'.
'sever'+'e'='SEVERE'
'to' acts as a link.
(Other definitions for severe that I've seen before include "Extremely difficult to endure" , "Harsh and exacting" , "Harsh and stern" , "Grave" , "Very plain in style or appearance" .)