Basic melt-in-the-middle cheese left by one Monsieur (9)
I believe the answer is:
elemental
'basic' is the definition.
(synonyms)
'melt-in-the-middle cheese left by one monsieur' is the wordplay.
'melt-in-the-middle' becomes 'el' (I can't justify this - if you can you should believe this answer much more).
'cheese' becomes 'Emmental' (Emmental is an example).
'left by' is a deletion indicator (word is left or abandoned by specified letters ).
'one monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'emmental' with 'm' taken out is 'emental'.
'el'+'emental'='ELEMENTAL'
Can you help me to learn more?
(Other definitions for elemental that I've seen before include "Nature spirit" , "Rising trend" , "key" , "of fire, say" , "Basic, rudimentary" .)