Odds on Leonard with sullen brilliance (9)
I believe the answer is:
splendour
'brilliance' is the definition.
('splendour' can be a synonym of 'brilliance')
'odds on leonard with sullen' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' is a charade indicator (letters next to each other).
'leonard' becomes 'len' (I've seen this in other clues).
'with' means one lot of letters go next to another.
'sullen' becomes 'dour' ('dour' can be a synonym of 'sullen').
'sp'+'len'+'dour'='SPLENDOUR'
(Other definitions for splendour that I've seen before include "Brilliance or pomp" , "Brilliant appearance" , "Magnificent appearance" , "Grandeur" , "Glory, brilliance" .)