Gunwales removed from passenger vessel in slip (3)
I believe the answer is:
err
'slip' is the definition.
(to err is to make an error)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for err that I've seen before include "Go wrong" , "'To . . . is human, to forgive divine'" , "Be inaccurate" , "Miscalculate" , "Transgress" .)