Used sneaky tactic filling in legal document (8)
I believe the answer is:
deployed
'used' is the definition.
(I've seen this before)
'sneaky tactic filling in legal document' is the wordplay.
'sneaky tactic' becomes 'ploy' (I've seen this in another clue).
'filling in' means one lot of letters goes inside another.
'legal document' becomes 'deed' (deed is a kind of legal document).
'ploy' inserted inside 'deed' is 'DEPLOYED'.
(Other definitions for deployed that I've seen before include "Established" , "Used" , "Placed troops in battle formation" , "Spread out and positioned for battle" , "Put" .)