Ghost in relative darkness (5)
I believe the answer is:
shade
I believe this is a double definition.
'ghost' is the first definition.
(I know that ghost can be written as shade)
'relative darkness' is the second definition.
(I've seen this before)
'in' is the link.
(Other definitions for shade that I've seen before include "Relative darkness" , "Colour; protection from sun" , "Quality of colour" , "Colour; slight amount" , "Slight colour difference" .)