Posh girl acquiring top-class place to eat? That’s a matter of opinion (9)
I believe the answer is:
debatable
'a matter of opinion' is the definition.
'posh girl acquiring top-class place to eat?' is the wordplay.
'posh girl acquiring' becomes 'deb' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'top class' becomes 'a' (top grade).
'place to eat?' becomes 'table'.
'deb'+'a'+'table'='DEBATABLE'
'that's' is the link.
(Other definitions for debatable that I've seen before include "subject to discussion" , "Dubious" , "Open to question" , "Moot" , "Open to argument" .)