Moderate king touring US city (5)
I believe the answer is:
relax
'moderate' is the definition.
(thesaurus)
'king touring us city' is the wordplay.
'king' becomes 'rex' (I've seen this before).
'touring' is an insertion indicator (touring can mean going round).
'us city' becomes 'la'.
'rex' going around 'la' is 'RELAX'.
(Other definitions for relax that I've seen before include "Don't worry" , "Slow down and unwind" , "Take it easy, don't be so tense" , "Unbend" , "Moderate" .)