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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'cheese' becomes 'Emmental' (Emmental is an example).
'left by' indicates named letters should be taken away (word is left or abandoned by specified letters ).
'one monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'emmental' with 'm' taken away 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" .)