He's excited by Alien film (5)
I believe the answer is:
sheet
'film' is the definition.
(I've seen this before)
'he's excited by alien' is the wordplay.
'excited' indicates an anagram.
'by' means one lot of letters go next to another.
'alien' becomes 'et' (extra terrestrial).
'hes' is an anagram of 'she'.
'she'+'et'='SHEET'
(Other definitions for sheet that I've seen before include "Piece of paper or bedding" , "Piece of linen, paper, etc" , "Bed cover" , "Item of bed linen like these" , "Mattress cover" .)