It’s awful when the ace turns up (3)
I believe the answer is:
bad
'it's awful' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bad that I've seen before include "Stinking" , "(Of food) decayed" , "not healthy" , "Of poor quality" , "Ill" .)