To growl at one's sweetheart is the wrong thing to do (3)
I believe the answer is:
err
'do' is the definition.
The answer and definition can be both to do with motion as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?
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 "Bungle" , "Transgress" , "'To . . . is human, to forgive divine'" , "'to... is human,to forgive divine'" , "Miscalculate" .)