One may fall into it by mistake (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'one may fall into it' is the wordplay.
I cannot quite see how this works, but
'fall' could be 'err' (I've seen this in another clue) and 'err' is found within the answer.
The remaining letters 'or' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for error that I've seen before include "Howler" , "A mistake or inaccuracy" , "heresy" , "Faux pas" , "Blunder, slip" .)