Spartan king in muddled mess (5)
I believe the answer is:
harsh
'spartan' is the definition.
(I've seen this in another clue)
'king in muddled mess' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'in' means one lot of letters goes inside another.
'muddled mess' becomes 'hash' (I've seen this in another clue).
'r' placed within 'hash' is 'HARSH'.
(Other definitions for harsh that I've seen before include "Jarring" , "Abrasive, rough or unkind" , "Punitive" , "Unpleasantly stern" , "Rough and disagreeable, maybe of voice" .)