Silky moth so fluttering (6)
I believe the answer is:
smooth
'silky' is the definition.
(similar in meaning)
'moth so fluttering' is the wordplay.
'fluttering' is an anagram indicator.
'moth'+'so'='mothso'
'mothso' is an anagram of 'SMOOTH'.
(Other definitions for smooth that I've seen before include "Not rough or coarse" , "Calm" , "without difficulty" , "Free from roughness in appearance or sound" , "Unlined, unwrinkled" .)