Legal or contrived musical term (7)
I believe the answer is:
allegro
'musical term' is the definition.
(I've seen this before)
'legal or contrived' is the wordplay.
'contrived' is an anagram indicator.
'legal'+'or'='legalor'
'legalor' is an anagram of 'ALLEGRO'.
(Other definitions for allegro that I've seen before include "Brisk movement or passage" , "To be performed in brisk style" , "Moving fast" , "In quick and lively style" , "Lively, quick (mus.)" .)