King remains reckless (4)
I believe the answer is:
rash
'reckless' is the definition.
('rash' can be a synonym of 'reckless')
'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 "Impulsive - eruption" , "Foolish; skin problem" , "Impulsive - skin condition" , "Impetuous; skin ailment" , "Ill-considered - outbreak" .)