Quatrain occasionally reflected demo, of course (9)
I believe the answer is:
naturally
'of course' is the definition.
(synonyms)
'quatrain occasionally reflected demo' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'reflected' shows that the letters should be reversed in order (letters reflected back).
'demo' becomes 'rally' (rally is a type of demonstration).
The alternating letters of 'quatrain' are 'utan'.
'utan' in reverse letter order is 'natu'.
'natu'+'rally'='NATURALLY'
(Other definitions for naturally that I've seen before include "In an artless way" , "Without artificial help" , "Unforced" , "as you might expect" , "Without affectation" .)