Ordinary novice supporting lady (6)
I believe the answer is:
normal
'ordinary' is the definition.
(thesaurus)
'novice supporting lady' is the wordplay.
'novice' becomes 'l' (abbreviation for Learner e.g. on UK cars).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'lady' becomes 'norma' (I've seen this before**).
'l' after 'norma' is 'NORMAL'.
(Other definitions for normal that I've seen before include "Customary" , "Accustomed" , "'Usual, standard (6)'" , "Ordinary, not extraordinary" , "Popular" .)