Nonsensical legal or musical term (7)
I believe the answer is:
Here is my best explanation:
'musical term' is the definition.
'nonsensical legal or' is the subsidiary indication.
'nonsensical' indicates an anagram.
'legalor' with letters rearranged gives 'allegro'.
(Other definitions for allegro that I've seen before include "Musical movement using a brisk speed", "At a brisk speed (music)", "Musically brisk", "Gaily, lively (music)", "Lively, quick (mus.)", "Austin", "Of music, fast and lively", "In quick time", "In a brisk and lively manner" and "quickly found in bars?".)