Document about device that's used (8)
I believe the answer is:
deployed
'used' is the definition.
(I've seen this before)
'document about device' is the wordplay.
'document' becomes 'deed' (deed is a kind of document).
'about' indicates putting letters inside.
'device' becomes 'ploy' (I've seen this before).
'deed' enclosing 'ploy' is 'DEPLOYED'.
'that's' acts as a link.
(Other definitions for deployed that I've seen before include "Moved troops into position for action" , "Put" , "Spread out and positioned for battle" , "Established" , "Placed troops in battle formation" .)