King remains hasty (4)
I believe the answer is:
rash
'hasty' is the definition.
(thesaurus)
'king remains' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'remains' becomes 'ash' (I've seen this before).
'r'+'ash'='RASH'
(Other definitions for rash that I've seen before include "Ill-considered - outbreak" , "Outbreak; hasty" , "Ill-considered - break-out" , "Impulsive, hasty" , "Flood" .)