Attaining less than zero, therefore really angry? (10)
I believe the answer is:
thunderous
'really angry?' is the definition.
(thunderous can mean extremely angry)
'attaining less than zero therefore' is the wordplay.
'attaining' means one lot of letters goes inside another (some letters get hold of or attain others).
'less than' becomes 'under' (eg under a certain number).
'zero' becomes 'O' (O resembles 0).
'therefore' becomes 'thus' (synonyms).
'under'+'o'='undero'
'undero' put inside 'thus' is 'THUNDEROUS'.
(Other definitions for thunderous that I've seen before include "Loud and rumbling" , "Deafening" .)