Chap screening pictures — one is ET (7)
I believe the answer is:
martian
'et' is the definition.
(a lifeform from Mars)
'chap screening pictures one' is the wordplay.
'chap' becomes 'man' (chap can mean a man).
'screening' is an insertion indicator (inserted letters are covered or screened by others).
'pictures' becomes 'art'.
'one' becomes 'i' (Roman numeral).
'art'+'i'='arti'
'man' placed around 'arti' is 'MARTIAN'.
'is' is the link.
(Other definitions for martian that I've seen before include "Green man from the Red Planet" , "Sci-fi character" , "seeing little green man?" , "Relating to a particular planet" , "One of H. G. Wells's invaders" .)