Penniless king in flowing robe (5)
I believe the answer is:
broke
'penniless' is the definition.
(I've seen this before)
'king in flowing robe' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'in' means one lot of letters goes inside another.
'flowing' is an anagram indicator.
'robe' anagrammed gives 'broe'.
'k' put inside 'broe' is 'BROKE'.
(Other definitions for broke that I've seen before include "Completely out of money" , "Moneyless" , "Lacking funds" , "Skint" , "Ordinal number" .)