Promotion of unstable comic realm (10)
I believe the answer is:
commercial
'promotion' is the definition.
(commercial is a kind of promotion)
'unstable comic realm' is the wordplay.
'unstable' is an anagram indicator.
'comic'+'realm'='comicrealm'
'comicrealm' is an anagram of 'COMMERCIAL'.
'of' acts as a link.
(Other definitions for commercial that I've seen before include "Profitable" , "It pushes" , "Greek island" , "Advertisement" , "Marketable" .)