Former politician remains controlling left (7)
I believe the answer is:
asquith
'former politician' is the definition.
(UK prime minister HH Asquith)
'remains controlling left' is the wordplay.
'remains' becomes 'ash' (remains of a fire).
'controlling' indicates putting letters inside.
'left' becomes 'quit' (I've seen this before).
'ash' enclosing 'quit' is 'ASQUITH'.
(Other definitions for asquith that I've seen before include "H H ---, Liberal PM" , "PM in 1914" , "old cabinet maker" , "Liberal prime minister, 1908 to 1916" , "PM 1908-16" .)