Rex remains impulsive (4)

I believe the answer is:

rash

'impulsive' is the definition.
(thesaurus)

'rex remains' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'remains' becomes 'ash' (I've seen this before).
'r'+'ash'='RASH'

(Other definitions for rash that I've seen before include "Headstrong" , "Impulsive - eruption" , "Impetuous; skin ailment" , "not considered" , "Ill-considered - break-out" .)

I've seen this clue in The Guardian.

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