Fill with optimism, energy and other things when upset (5)
I believe the answer is:
elate
'fill with optimism' is the definition.
(I've seen this before)
'energy and other things when upset' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and other things' becomes 'etal' (I've seen this before).
'when upset' says the letters should be written backwards (in a down clue - set, written, up).
'etal' reversed gives 'late'.
'e'+'late'='ELATE'
(Other definitions for elate that I've seen before include "Bring joy" , "Make joyful and proud" , "Exhilarate, put in high spirits" , "Fill with optimism" , "Make ecstatically happy" .)