Knockabout fun as her ploy goes wrong (9)
I believe the answer is:
horseplay
'knockabout fun' is the definition.
(horseplay is rough play)
'as her ploy goes wrong' is the wordplay.
'goes wrong' indicates an anagram.
'as'+'her'+'ploy'='asherploy'
'asherploy' with letters rearranged gives 'HORSEPLAY'.
(Other definitions for horseplay that I've seen before include "Rough boisterous action" , "Boisterous antics" , "Messing about (as a steed?)" , "clowning around" , "Rough or boisterous activity" .)