Briefly legal (5)
I believe the answer is:
legit
'legal' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for legit that I've seen before include "In accordance with the law (abbr)" , "true, in a word?" , "Right" , "on the level" , "acceptable" .)