Slip off edge of ferry (3)
I believe the answer is:
err
'slip off edge' is the definition.
'err' can be an answer for 'slip' (to err is to make an error). I am not certain of the 'off edge' bit.
'of ferry' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ERR' can be found hidden inside 'ferry'.
Can you help me to learn more?
(Other definitions for err that I've seen before include "Make mistake in Canberra" , "Blunder" , "Miss the mark" , "'To . . . is human, to forgive divine'" , "Make mistakes" .)