Make a typo, say (3)
I believe the answer is:
err
'make a typo say' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for err that I've seen before include "Transgress" , "Drop a clanger" , "do something you shouldn't" , "Go astray" , "Stray from the straight and narrow" .)