Bad temper, no energy - stern magnificence (9)
I believe the answer is:
splendour
'magnificence' is the definition.
(I know that magnificence can be written as splendour)
'bad temper no energy stern' is the wordplay.
'bad temper' becomes 'spleen' (informal term for bad temper).
'no' indicates named letters should be taken away.
'energy' becomes 'e' (physics symbol as in E=mc2).
'stern' becomes 'dour' (I've seen this in another clue).
'spleen' with 'e' removed is 'splen'.
'splen'+'dour'='SPLENDOUR'
(Other definitions for splendour that I've seen before include "Brilliance or pomp" , "Magnificence, brilliance" , "Grandeur" , "Brilliant appearance" , "show" .)