Don't pass round large flounder (5)
I believe the answer is:
flail
'flounder' is the definition.
'don't pass round large' is the wordplay.
'don't pass' becomes 'fail' (as in failing to pass an exam).
'round' is an insertion indicator.
'large' becomes 'l'.
'fail' enclosing 'l' is 'FLAIL'.
(Other definitions for flail that I've seen before include "Weapon; swing wildly" , "Threshing device" , "Thresher; wave helplessly about" , "Wave about wildly" , "thrash around" .)