Key point about sci-fi is not to start getting cross (8)
I believe the answer is:
crucifix
'cross' is the definition.
(crucifix is a kind of cross)
'key point about sci-fi is not to start' is the wordplay.
'key point' becomes 'crux' (I've seen this before).
'about' indicates putting letters inside.
'is not to start' means to remove the first letter.
'sci'+'fi'='scifi'
'scifi' with its first letter removed is 'cifi'.
'crux' placed around 'cifi' is 'CRUCIFIX'.
'getting' is the link.
(Other definitions for crucifix that I've seen before include "Necklace popular with Christians" , "Christian symbol" , "Religious cross" , "The Christian cross" , "Representation of the cross on which Jesus died" .)