Cover story? (3)
'cover story?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alibi that I've seen before include "Defence in court" , "Defence against accusation - was somewhere else" , "It shows one wasn't at crime scene" , "Defence against charge - was seen elsewhere" , "Court evidence showing one was elsewhere" .)
(Other definitions for fib that I've seen before include "Little white lie" , "Story" , "Small lie" , "minor piece of fiction" , "Petty falsehood" .)