Poor sci-fi turned out to be dull (9)
I believe the answer is:
soporific
'be dull' is the definition.
(similar in meaning)
'poor sci-fi turned out' is the wordplay.
'turned out' indicates anagramming the letters (turn out can mean to organise).
'poor'+'sci'+'fi'='poorscifi'
'poorscifi' is an anagram of 'SOPORIFIC'.
'to' is the link.
(Other definitions for soporific that I've seen before include "Inducing drowsiness or sleep" , "Sedative" , "making one nod" , "Sleep-inducing" , "Causing sleepiness" .)