Tiniest note accompanies letter from abroad that's returned (7)
I believe the answer is:
minimum
'tiniest note accompanies letter from abroad that's returned' is the definition.
I can't judge whether this definition defines the answer.
'note accompanies letter from abroad that's returned' is the wordplay.
'note' becomes 'minim' (type of musical note).
'accompanies' is a charade indicator (letters next to each other).
'letter from abroad' becomes 'mu' (letter of the Greek alphabet).
'that's returned' shows that the letters should be reversed in order.
'mu' reversed gives 'um'.
'minim'+'um'='MINIMUM'
Can you help me to learn more?
(Other definitions for minimum that I've seen before include "Bottom" , "Nadir" , "Least possible quantity" , "Lowest value" , "The least of smallest amount possible" .)