Under yours truly, every king initially submissive (4)
I believe the answer is:
meek
'submissive' is the definition.
(thesaurus)
'under yours truly every king initially' is the wordplay.
'under' says to put letters next to each other.
'yours truly' becomes 'me'.
'initially' suggests taking the first letters.
The initial letters of 'every king' is 'ek'.
'me' next to 'ek' is 'MEEK'.
(Other definitions for meek that I've seen before include "Very docile" , "Gentle and submissive" , "Humble and mild" , "Quiet" , "Gentle, timid" .)