Nonsensical legal or musical term (7)
I believe the answer is:
allegro
'musical term' is the definition.
(I've seen this before)
'nonsensical legal or' is the wordplay.
'nonsensical' is an anagram indicator.
'legal'+'or'='legalor'
'legalor' with letters rearranged gives 'ALLEGRO'.
(Other definitions for allegro that I've seen before include "Brisk movement or passage" , "To be performed in brisk style" , "(In music) a brisk and lively tempo" , "Austin" , "(In music) briskly" .)