With a look the little devil may injure one (6)
I believe the answer is:
impair
'injure one' is the definition.
'impair' can be an answer for 'injure' (I've seen this in another clue). I'm not certain of the 'one' bit.
'with a look the little devil may' is the wordplay.
'with' is an insertion indicator.
'a look' becomes 'air' (something might have a certain air or look).
'the little' means to remove the last letter (the word is not quite complete).
'devil' becomes 'imp' (devilish creature).
'may' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'air' with its final letter taken off is 'ai'.
'imp'+'r'='impr'
'ai' put within 'impr' is 'IMPAIR'.
Can you help me to learn more?
(Other definitions for impair that I've seen before include "Hobble" , "Damage or handicap" , "'Mar, damage (6)'" , "Hinder" , "Make worse or less effective" .)