When most watch proper sci-fi film on writer's tablet (5,4)
I believe the answer is:
prime time
'when most watch' is the definition.
(most popular time for TV viewing)
'proper sci-fi film on writer's tablet' is the wordplay.
'proper' becomes 'prim' ('prim' can be similar in meaning to 'proper'**).
'sci-fi film' becomes 'et' (I've seen this in other clues).
'on' says to put letters next to each other.
'writer's' becomes 'im' ('writer's' could mean 'the crossword setter is').
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'prim'+'et'+'im'+'e'='PRIME TIME'
(Other definitions for prime time that I've seen before include "when most view" , "when most people are turned on" , "When viewing figures are highest" , "Peak viewing period" , "For monks perhaps around dawn" .)