Remain outside a coach as required by law (9)
I believe the answer is:
statutory
'required by law' is the definition.
(I've seen this before)
'remain outside a coach' is the wordplay.
'remain' becomes 'stay' (synonyms).
'outside' means one lot of letters goes inside another.
'a coach' becomes 'tutor' ('tutor' can be a synonym of 'coach').
'stay' enclosing 'tutor' is 'STATUTORY'.
'as' is the link.
(Other definitions for statutory that I've seen before include "Required by written law" , "According to law" , "Relating to written law" , "Prescribed or authorised by law" , "Try-outs at being required by law" .)