Medium Ravel composition is a wonderful thing (6)
I believe the answer is:
marvel
'a wonderful thing' is the definition.
(I've seen this before)
'medium ravel composition' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'composition' indicates an anagram (letters arranged or composed in a different order).
'ravel' with letters rearranged gives 'arvel'.
'm'+'arvel'='MARVEL'
'is' acts as a link.
(Other definitions for marvel that I've seen before include "Express wonder" , "Feel sense of wonder and awe" , "'Wonder, miracle (6)'" , "Be filled with surprise or wonder" , "Wonder at, be amazed" .)