Member got off by unknown feature of law (8)
I believe the answer is:
legality
'feature of law' is the definition.
(legality is following the law)
'member got off by unknown' is the wordplay.
'member' becomes 'leg' (leg is a kind of member).
'got off' becomes 'alit' ('alight' can mean to get off a vehicle).
'by' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'leg'+'alit'+'y'='LEGALITY'
(Other definitions for legality that I've seen before include "Soundness" , "State of lawfulness" , "State of being allowed in law" .)