Greatly upset about hot, sleepy condition (8)
I believe the answer is:
lethargy
'sleepy condition' is the definition.
(condition of being sleepy or tired)
'greatly upset about hot' is the wordplay.
'upset' indicates anagramming the letters.
'about' indicates putting letters inside.
'hot' becomes 'h' (abbreviation eg on taps).
'greatly' is an anagram of 'letargy'.
'letargy' going around 'h' is 'LETHARGY'.
(Other definitions for lethargy that I've seen before include "Inertia" , "Lassitude, sluggishness" , "Stringed instrument" , "'Lassitude, langour (8)'" , "Lack of vitality or energy" .)