Scouts, as a rule, messed up part of basic training (7,6)
I believe the answer is:
assault course
'part of basic training' is the definition.
(exercise training)
'scouts as a rule messed up' is the wordplay.
'messed up' indicates anagramming the letters.
'scouts'+'as'+'a'+'rule'='scoutsasarule'
'scoutsasarule' anagrammed gives 'ASSAULT COURSE'.
(Other definitions for assault course that I've seen before include "Test of fitness and endurance" , "Training exercise (mil.)" , "Arduous training for soldiers" , "for cadets in training?" , "Soldier training circuit" .)