Reason for mischief-maker to postpone session (8)
I believe the answer is:
prorogue
'postpone session' is the definition.
(to discontinue eg a parliamentary session)
'reason for mischief-maker' is the wordplay.
'reason for' becomes 'pro'.
'mischief maker' becomes 'rogue'.
'pro'+'rogue'='PROROGUE'
'to' acts as a link.
(Other definitions for prorogue that I've seen before include "Discontinue meeting" , "Suspend, postpone" , "Suspend meeting" , "Discontinue (a session of parliament)" , "Stop meeting" .)