Elm is so amazingly flexible (7)
I believe the answer is:
lissome
'flexible' is the definition.
(I've seen this before)
'elm is so amazingly' is the wordplay.
'amazingly' indicates anagramming the letters.
'elm'+'is'+'so'='elmisso'
'elmisso' anagrammed gives 'LISSOME'.
(Other definitions for lissome that I've seen before include "Moving supply" , "pliable" , "So smile, being agile and lithe" , "Slim, supple, graceful" , "Thin - supple - graceful" .)