Excuse from Crystal, "I bilked" (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'from crystal i bilked' is the wordplay.
'from' indicates the answer is hidden within the clue.
'ALIBI' can be found hidden inside 'crystal i bilked'.
(Other definitions for alibi that I've seen before include "Timely excuse" , "Defence used by an accused person" , "that gets him off the hook?" , "Timely get-out" , "can offer a valid defence against most charges" .)