Break-dancer on arms (8)
I believe the answer is:
ordnance
'arms' is the definition.
(ordnance is a kind of arms)
'break-dancer on' is the wordplay.
'break' indicates an anagram.
'dancer'+'on'='danceron'
'danceron' is an anagram of 'ORDNANCE'.
(Other definitions for ordnance that I've seen before include "Car on end (anag) - artillery" , "Heavy guns and military supplies" , "regulation" , "Survey may follow this" , "Military supplies and materials" .)