Knock down mother reversing out of control (4)
I believe the answer is:
amok
'out of control' is the definition.
(I've seen this before)
'knock down mother reversing' is the wordplay.
'knock down' becomes 'KO' (knock out, as in boxing).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'reversing' says the letters should be written backwards.
'ko'+'ma'='koma'
'koma' written backwards gives 'AMOK'.
(Other definitions for amok that I've seen before include "Frenziedly" , "Frenzied, running out of control" , "Must run it" , "Uncontrolled or murderous way to run!" , "Berserk" .)