King going outside city to unwind (5)
I believe the answer is:
relax
'to unwind' is the definition.
('relax' can be a synonym of 'unwind')
'king going outside city' is the wordplay.
'king' becomes 'rex' ('king' in Latin).
'going outside' means one lot of letters goes inside another (the word goes on the outside of inserted letters).
'city' becomes 'LA' (Los Angeles).
'rex' enclosing 'la' is 'RELAX'.
(Other definitions for relax that I've seen before include "Slow down and unwind" , "Moderate" , "Rest" , "Put your feet up" , "Wind down" .)