Self-justification in fib I laughed about (5)
I believe the answer is:
alibi
'self-justification' is the definition.
(I know that self-justification can be written as alibi)
'in fib i laughed about' is the wordplay.
'in' says the answer is hidden in the clue.
'about' is a reversal indicator.
'ibila' is hidden in the letters of 'fib i laughed'.
'ibila' backwards is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Sort of explanation" , "Courtroom evidence" , ""suspect's story"" , "Defence against accusation - was seen elsewhere at the time" , "Legal excuse" .)