At first Michael Caine ruined movies (6)
I believe the answer is:
cinema
'movies' is the definition.
(I've seen this before)
'at first michael caine ruined' is the wordplay.
'at first' says to take the initial letters.
'ruined' is an anagram indicator.
The first letter of 'michael' is 'm'.
'm'+'caine'='mcaine'
'mcaine' with letters rearranged gives 'CINEMA'.
(Other definitions for cinema that I've seen before include "Film industry" , "Movies, pictures" , "Modern art" , "Films, collectively" , "Movie theatre" .)