On a charge this might well be sinister (4)
I believe the answer is:
bend
'on a charge this might well be sinister' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'on a charge this might well be' is the wordplay.
'on' means one lot of letters go next to another.
'a charge this might well' becomes 'nd' (I can't justify this - if you can you should give a lot more credence to this answer).
'nd' after 'be' is 'BEND'.
Can you help me to learn more?
(Other definitions for bend that I've seen before include "Country; bird" , "elbow" , "Crouch" , "Crook" , "Corner" .)