Exercise right to sit on panel — that's a crime (7)
I believe the answer is:
Here is my best explanation:
'a crime' is the definition.
(perjury is a kind of crime)
'exercise right to sit on panel' is the wordplay.
'exercise' becomes 'pe'.
'right' becomes 'r' (common abbreviation).
'to sit on' is a charade indicator (letters next to each other) (in a down clue, some letters go on top of others).
'panel' becomes 'jury' ('jury' can be a synonym of 'panel').
'that's' acts as a link.
(Other definitions for perjury that I've seen before include "False witness", "Telling lies in court under oath", "False swearing", "The telling of a lie under oath", "Offence in court", "Crime of lying under oath", "The crime of lying under oath", "Lying in court", "Offence of lying under oath" and "Lying under oath".)