Alien was seen here and came in after mutating (6)
I believe the answer is:
cinema
'alien was seen here' is the definition.
(reference to the film Alien)
'came in after mutating' is the wordplay.
'after mutating' indicates anagramming the letters.
'came'+'in'='camein'
'camein' anagrammed gives 'CINEMA'.
'and' is the link.
(Other definitions for cinema that I've seen before include "Movie-house" , "Film industry" , "Film theatre" , "Picture house" , "(Building for) film" .)