Plants displayed round a monarch's houses (7)
I believe the answer is:
palaces
'monarch's houses' is the definition.
'plants displayed round a' is the wordplay.
'plants' becomes 'places' (plant and place can both mean to position).
'displayed round' is an insertion indicator.
'places' going around 'a' is 'PALACES'.
(Other definitions for palaces that I've seen before include "Royal residences" , "kings and queens here?" , "Offical residences of sovereigns" .)