A very bad act in final game is really stupid (7)
I believe the answer is:
asinine
'really stupid' is the definition.
'asinine' can be an answer for 'stupid' (I've seen this before). I'm not sure about the 'really' bit.
'a very bad act in final game' is the wordplay.
'a very bad act' becomes 'asin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'final' indicates one should take the final letters.
The last letter of 'game' is 'e'.
'asin'+'in'+'e'='ASININE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for asinine that I've seen before include "simple" , "Silly like a donkey" , "Stupid (as a donkey?)" , "Daft" , "Extremely stupid or foolish" .)