He could be angriest (5,3)
I believe the answer is:
inert gas
'he' is the definition.
(symbol for Helium)
'could be angriest' is the wordplay.
'could be' indicates anagramming the letters.
'angriest' anagrammed gives 'INERT GAS'.
(Other definitions for inert gas that I've seen before include "Eg, helium or argon" , "Such as neon or xenon" , "One expects no reaction from such" , "He, amongst others" , "Passive element in the helium group" .)