Feature a group of extremists seizing power (6)
I believe the answer is:
aspect
'feature' is the definition.
(feature can mean a quality or aspect of something)
'a group of extremists seizing power' is the wordplay.
'group of extremists' becomes 'sect' (as in a religious sect).
'seizing' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'sect' enclosing 'p' is 'spect'.
'a'+'spect'='ASPECT'
(Other definitions for aspect that I've seen before include "'Scene, view, vista (6)'" , "Characteristic to be considered" , "Vista, panorama" , "View, panorama" , "'View, perspective (6)'" .)