No good in the sack, gutted adult admitted (3)
I believe the answer is:
bad
'no good' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for bad that I've seen before include "Harmful" , "SECOND HALF OF 16" , "Ill" , "Mouldy, off" , "(Of food) decayed" .)