Key material originally redacted (4)
I believe the answer is:
isle
'key' is the definition.
(I've seen this before)
'material originally redacted' is the wordplay.
'material' becomes 'lisle' (lisle is a kind of material).
'originally redacted' means to remove the first letter (the origin of the word is redacted).
'lisle' with its first letter removed is 'ISLE'.
(Other definitions for isle that I've seen before include "eg Thanet" , "Piece of land in water" , "Dogs, for example" , "Skye, for instance" , "Dogs, perhaps" .)