Confines overambitious guards (5)
I believe the answer is:
ambit
'confines' is the definition.
(I've seen this before)
'overambitious guards' is the wordplay.
'guards' indicates the answer is hidden within the clue.
'AMBIT' is hidden within 'overambitious'.
(Other definitions for ambit that I've seen before include "Circuit, compass" , "Circumference" , "Scope, extent, or bounds of something" , "Compass, scope" , "Range - surroundings" .)