Treat from Filbert, mostly the best compiler (8)
I believe the answer is:
noisette
'treat from filbert' is the definition.
The definition suggests a singular noun which matches the answer.
'mostly the best compiler' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'the best' becomes 'noi' (resembles No. 1, the best).
'compiler' becomes 'setter' (both can mean a writer of crosswords).
'noi'+'setter'='noisetter'
'noisetter' with its final letter removed is 'NOISETTE'.
Can you help me to learn more?
(Other definitions for noisette that I've seen before include "Hazelnut chocolate" , "Piece of rolled lamb (flavoured with hazel nuts?)" , "Chocolate made with hazelnuts" , "Small piece of boneless lamb" , "Rolled piece of lamb; nutty sweet" .)