Prepare (a disk for data) (6)
I believe the answer is:
format
'prepare a disk for data' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for format that I've seen before include "Arrangement or lay-out, say of book" , "Style or arrangement of a TV programme" , "Way of presentation" , "Arrangement (data)" , "Prepare (a disk) for data" .)