Kingly leaders of red or yellow arum lilies (5)
I believe the answer is:
royal
'kingly' is the definition.
(I've seen this before)
'leaders of red or yellow arum lilies' is the wordplay.
'leaders of' suggests taking the first letters.
The initial letters of 'red or yellow arum lilies' is 'ROYAL'.
(Other definitions for royal that I've seen before include "Of queens, say" , "Queen, say?" , "one in palace?" , "Befitting a monarch" , "Relating to kings/queens" .)