Legal or otherwise, it's a lively movement (7)
I believe the answer is:
allegro
'a lively movement' is the definition.
'allegro' can be an answer for 'movement' (I have seen 'Brisk movement' mean 'allegro' so perhaps 'movement' could also mean 'allegro'). I'm not certain of the 'a lively' bit.
'legal or otherwise' is the wordplay.
'otherwise' indicates anagramming the letters.
'legal'+'or'='legalor'
'legalor' with letters rearranged gives 'ALLEGRO'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for allegro that I've seen before include "'At a brisk speed, musically (7)'" , "In quick and lively style" , "'Brisk and lively, musically (7)'" , "Scored at a brisk speed" , "At a brisk speed (music)" .)