From the devil, a look can harm (6)
I believe the answer is:
impair
'harm' is the definition.
(I've seen this before)
'from the devil a look' is the wordplay.
I cannot really see how this works, but
'devil' could be 'imp' (devilish creature) and 'imp' is found within the answer.
'a' is found in the answer.
This explanation may well be incorrect...
'can' is the link.
Can you help me to learn more?
(Other definitions for impair that I've seen before include "Make worse or less effective" , "'Mar, damage (6)'" , "Weaken, damage" , "Hinder" , "Hurt or handicap" .)