Pity he's so confused about hard theory (10)
I believe the answer is:
hypothesis
'theory' is the definition.
(synonyms)
'pity he's so confused about hard' is the wordplay.
'confused' indicates anagramming the letters.
'about' indicates putting letters inside.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'pity'+'hes'+'so'='pityhesso'
'pityhesso' anagrammed gives 'hypotesis'.
'hypotesis' going around 'h' is 'HYPOTHESIS'.
(Other definitions for hypothesis that I've seen before include "Proposed explanation" , "Speculation" , "conjecture" , "Guess" , "Supposition" .)