Queen depicted upside down? Landlord's no monarchist (10)
I believe the answer is:
republican
'no monarchist' is the definition.
'queen depicted upside down? landlord's' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'depicted upside down?' is a reversal indicator.
'landlord' becomes 'publican' (I've seen this before).
'er' in reverse letter order is 're'.
're'+'publican'='REPUBLICAN'
(Other definitions for republican that I've seen before include "in foreign party" , "anti-monarchist" , "American in party" , "American politician" , "Member of US political party" .)