One forgets a cinema's changed (8)
I believe the answer is:
amnesiac
'one forgets' is the definition.
(I've seen this before)
'a cinema's changed' is the wordplay.
'changed' is an anagram indicator.
'cinemas' is an anagram of 'mnesiac'.
'a'+'mnesiac'='AMNESIAC'
(Other definitions for amnesiac that I've seen before include "One quite forgets" , "Person suffering clinical loss of money" , "Memory-loss sufferer" , "Unaware of past" , "I have difficulty retaining" .)