Missile's course (6)
I believe the answer is:
vector
'missile's course' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vector that I've seen before include "Carrier of infection and course of a missile" , "Covert sort of mathematical quantity" , "Covert sort of maths quantity" , "Quantity having direction as well as magnitude - covert (anag)" , "Disease carrier; mathematical quantity" .)