Great sandwich must have a filling (6)
I believe the answer is:
superb
'great' is the definition.
(I know that great can be written as superb)
'sandwich must have a filling' is the wordplay.
I cannot really understand how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
'sandwich' could be 'sub' (sub is a kind of sandwich) and 'sub' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for superb that I've seen before include "Admirable" , "choice" , "Excellent, magnificent" , "First-rate" , "Brilliant, fantastic" .)