Basic mistake, at first, to leave cheese around the French (9)
I believe the answer is:
elemental
'basic' is the definition.
(synonyms)
'mistake at first to leave cheese around the french' is the wordplay.
'mistake' indicates anagramming the letters.
'at' means one lot of letters go next to another.
'first to' suggests taking the first letters.
'around' indicates the central letters.
'the french' becomes 'mental' (I can't explain this - if you can you should give a lot more credence to this answer).
The centre of 'cheese' is 'ee'.
The initial letter of 'leave' is 'l'.
'l' put after 'ee' is 'eel'.
'eel' is an anagram of 'ele'.
'ele'+'mental'='ELEMENTAL'
(Other definitions for elemental that I've seen before include "being like Mercury?" , "Of nature's powers" , "key" , "Nature spirit" , "Basic, primitive" .)