On the side or behind demonstration (9)
I believe the answer is:
laterally
'on the side' is the definition.
(laterally means on the side)
'behind demonstration' is the wordplay.
'behind' becomes 'late' (as in behind schedule).
'demonstration' becomes 'rally' (I've seen this in another clue).
'late'+'rally'='LATERALLY'
'or' acts as a link.
(Other definitions for laterally that I've seen before include "Approaching the side" , "Sideways" .)