Pity he's so worked up about hot theory (10)
I believe the answer is:
hypothesis
'theory' is the definition.
(I know that theory can be written as hypothesis)
'pity he's so worked up about hot' is the wordplay.
'worked up' is an anagram indicator.
'about' says to put letters next to each other.
'hot' becomes 'h' (abbreviation eg on taps).
'pity'+'hes'+'so'='pityhesso'
'pityhesso' anagrammed gives 'ypothesis'.
'ypothesis' after 'h' is 'HYPOTHESIS'.
(Other definitions for hypothesis that I've seen before include "Supposition" , "conjecture" , "idea" , "Guess" , "Speculation" .)