Figure of Conservative previously in retreat (7)
I believe the answer is:
decagon
'figure' is the definition.
(type of geometrical figure)
'conservative previously in retreat' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'previously' becomes 'ago' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'retreat' becomes 'den' (both can mean a lair or hideout).
'c'+'ago'='cago'
'cago' put into 'den' is 'DECAGON'.
'of' acts as a link.
(Other definitions for decagon that I've seen before include "Go, dance on geometric figure with ten sides" , "A polygon -- eg can-do (anag)" , "Plane shape" , "Ten-sided shape" , "Figure with 10 sides" .)