Mistake stretching US A&E department? (3)
I believe the answer is:
err
'mistake' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for err that I've seen before include "Blunder" , "Be mistaken" , "Leave the straight and narrow" , "Be inaccurate" , "Go wrong" .)