Fold note in container (6)
I believe the answer is:
crease
'fold' is the definition.
(I know that fold can be written as crease)
'note in container' is the wordplay.
'note' becomes 're' (as in do-re-mi).
'in' means one lot of letters goes inside another.
'container' becomes 'case' (case is a kind of container).
're' placed into 'case' is 'CREASE'.
(Other definitions for crease that I've seen before include "Furrow, wrinkle" , "Where the batsman stands" , "Fold - wrinkle" , "Wrinkle on the cricket pitch?" , "Cricket line; wrinkle" .)