So in sci-fi film, boy is mysterious (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'so in sci-fi film boy' is the wordplay.
'in' indicates putting letters inside.
'sci-fi film' becomes 'et' (I've seen this in other clues).
'boy' becomes 'eric' (I've seen this before**).
'so' inserted into 'et' is 'esot'.
'esot'+'eric'='ESOTERIC'
'is' acts as a link.
(Other definitions for esoteric that I've seen before include "Mystic" , "Secret; taught to few" , "Other-worldly" , "exclusive" , "Secret, private" .)