A posh cheese and drink Maria’s left: it may be on the rocks (9)
I believe the answer is:
aubrietia
'it may be on the rocks' is the definition.
(plant which often grows among rocks)
'a posh cheese and drink maria's left' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'cheese' becomes 'brie' (Brie is an example).
'and' means one lot of letters go next to another.
'drink maria' becomes 'ait' (I am not sure about this - if you are sure you should believe this answer much more).
'left' is a reversal indicator.
'ait' written backwards gives 'tia'.
'a'+'u'+'brie'+'tia'='AUBRIETIA'
(Other definitions for aubrietia that I've seen before include "Flower for rock gardens" , "Plant producing" .)