Stratagems Rex employs (5)
I believe the answer is:
ruses
'stratagems' is the definition.
(I've seen this in another clue)
'rex employs' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'employs' becomes 'uses' ('use' can be a synonym of 'employ').
'r'+'uses'='RUSES'
(Other definitions for ruses that I've seen before include "Tactics" , "cunning tricks" , "Stratagem designed to deceive" , "Dodges" , "Clever plans designed to deceive" .)