Harsh to lop off eel's head (6)
I believe the answer is:
severe
'harsh' is the definition.
(I've seen this before)
'lop off eel's head' is the wordplay.
'lop off' becomes 'sever' (I've seen this in another clue).
'head' says to take the initial letters.
The initial letter of 'eels' is 'e'.
'sever'+'e'='SEVERE'
'to' acts as a link.
(Other definitions for severe that I've seen before include "Unsparing and uncompromising" , "Harsh, rigorous" , "Drastic" , "Intense; harsh" , "Forbidding in action or appearance" .)