Reichstag group's hostile attitude (5)
I believe the answer is:
aggro
'hostile attitude' is the definition.
(aggression)
'reichstag group's' is the wordplay.
'AGGRO' is hidden within the letters.
However, I am not sure how this is indicated.
(Other definitions for aggro that I've seen before include "Difficulties (informal)" , "Slang for deliberate trouble-making" , "Hostility" , "Violent disturbance" , "Problems and difficulties" .)