Carefree Berliner is mistakenly keeping performance short (13)
I believe the answer is:
irresponsible
'carefree' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for irresponsible that I've seen before include "Unreliable" , "Negligent" , "Wantonly careless" , "Showing a lack of care for consequences" , "Reprehensibly careless" .)