Essential as scenery changes (9)
I believe the answer is:
necessary
'essential' is the definition.
(similar in meaning)
'as scenery changes' is the wordplay.
'changes' indicates anagramming the letters.
'as'+'scenery'='asscenery'
'asscenery' is an anagram of 'NECESSARY'.
(Other definitions for necessary that I've seen before include "Required to be done" , "Essential, needed" , "Indespensable, essential" , "Inescapable" , "the money?" .)