Compose a sonata piece that's mellow (6)
I believe the answer is:
season
'mellow' is the definition.
(thesaurus)
'compose a sonata piece' is the wordplay.
'piece' says the answer is hidden in the clue.
'SEASON' is hidden within 'compose a sonata'.
'that's' is the link.
(Other definitions for season that I've seen before include "Quarter of the year" , "eg Winter" , "temper" , "Time of 9" , "Add taste to; part of year" .)