Came in to improve building (6)
I believe the answer is:
cinema
'building' is the definition.
(cinema is a kind of building)
'came in to improve' is the wordplay.
'to improve' indicates anagramming the letters.
'came'+'in'='camein'
'camein' anagrammed gives 'CINEMA'.
(Other definitions for cinema that I've seen before include "Screening" , "Movies, pictures" , "Movie theatre" , "multiplex, perhaps" , "place for pictures" .)