Musical term or legal nonsense (7)
I believe the answer is:
allegro
'musical term' is the definition.
(I've seen this before)
'or legal nonsense' is the wordplay.
'nonsense' indicates an anagram.
'or'+'legal'='orlegal'
'orlegal' is an anagram of 'ALLEGRO'.
(Other definitions for allegro that I've seen before include "Old car" , "Fast (mus.)" , "Quick (in music)" , "Brisk and lively tempo, musically" , "In quick and lively style" .)