Progressive revised bill covering historical period (7)
I believe the answer is:
liberal
'progressive' is the definition.
('liberal' can be a synonym of 'progressive')
'revised bill covering historical period' is the wordplay.
'revised' indicates an anagram.
'covering' means one lot of letters goes inside another.
'historical period' becomes 'era' (I've seen this before).
'bill' with letters rearranged gives 'libl'.
'libl' placed around 'era' is 'LIBERAL'.
(Other definitions for liberal that I've seen before include "Not strict" , "Braille (anag.)" , "Ample; former politician" , "Bountiful" , "Old politician" .)