Some sex actually is spot on (5)
I believe the answer is:
exact
'spot on' is the definition.
(spot on means exactly right)
'some sex actually is' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'EXACT' is hidden within 'sex actually is'.
(Other definitions for exact that I've seen before include "Completely accurate" , "Strict" , "accurate in all details" , "On the nail" , "Faithful" .)