Cloaked in scarlet, one's king is imperilled (6)
I believe the answer is:
risked
'is imperilled' is the definition.
(put at risk)
'cloaked in scarlet one's king' is the wordplay.
'cloaked in' is an insertion indicator.
'scarlet' becomes 'red' (synonyms).
'one's' becomes 'I's' (Roman numeral).
'king' becomes 'K' (abbreviation for king in chess).
'is'+'k'='isk'
'red' enclosing 'isk' is 'RISKED'.
(Other definitions for risked that I've seen before include "Incurred the chance of misfortune" , "Took a chance, gambled" , "Exposed to danger or loss" .)