Get the wrong end of the stick (13)
I believe the answer is:
misunderstand
'get the wrong end of the stick' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for misunderstand that I've seen before include "Wrongly grasp" , "Fail to follow" , "Get wrong idea of" , "Make little of" , "Get the wrong end of the stick" .)