Self-assured, virtually laid back, private (12)
I believe the answer is:
confidential
'private' is the definition.
('confidential' can be similar in meaning to 'private')
'self-assured virtually laid back' is the wordplay.
'self-assured' becomes 'confident' ('confident' can be similar in meaning to 'self-assured').
'virtually' means to remove the last letter (all but the last letter).
'back' shows that the letters should be reversed in order.
'laid' with its last letter taken away is 'lai'.
'lai' in reverse letter order is 'ial'.
'confident'+'ial'='CONFIDENTIAL'
(Other definitions for confidential that I've seen before include "not expecting publication" , "Classified" , "Intended to be kept secret" , "Graham Greene's agent" , "Like T.S. Eliot's clerk" .)