The D in CD-ROM (4)
I believe the answer is:
disc
'the d in cd-rom' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for disc that I've seen before include "something moving in the spine" , "Flat, circular plate - record" , "Compact ---; slipped ---" , "Circular flat structure in the spine" , "counter" .)