Render fern with poetic flawlessness (10)
I believe the answer is:
perfection
'flawlessness' is the definition.
(I know that flawlessness can be written as perfection)
'render fern with poetic' is the wordplay.
'render' indicates anagramming the letters (I've seen 'rendered' mean this).
'with' is a charade indicator (letters next to each other).
'fern' put after 'poetic' is 'poeticfern'.
'poeticfern' is an anagram of 'PERFECTION'.
(Other definitions for perfection that I've seen before include "Highest state" , "Faultlessness" .)