Container clue designed to cover a note (7)
I believe the answer is:
capsule
'container' is the definition.
(I have seen 'Small container ' mean 'capsule' so perhaps 'container' could also mean 'capsule')
'clue designed to cover a note' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'to cover' is an insertion indicator.
'note' becomes 'ps' (a PS or postscript in a letter).
'clue' anagrammed gives 'cule'.
'a'+'ps'='aps'
'cule' enclosing 'aps' is 'CAPSULE'.
(Other definitions for capsule that I've seen before include "Small compartment for astronauts" , "Type of pill" , "Gelatin case (for medication)" , "Space crew return in it" , "Lozenge - space vehicle" .)