Private is sure of themselves and laid back, in short (12)
I believe the answer is:
confidential
'private' is the definition.
(similar in meaning)
'sure of themselves and laid back in short' is the wordplay.
I cannot quite see how this works, but
'sure' could be 'confident' (similar in meaning) and 'confident' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for confidential that I've seen before include "Intended to be kept secret" , "Graham Greene's agent" , "Inflicted on a (anag) - private" , "not expecting publication" , "Classified" .)