Required fellow nowadays to back politician (9)
I believe the answer is:
mandatory
'required' is the definition.
(I know that required can be written as mandatory)
'fellow nowadays to back politician' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'to back' says the letters should be written backwards.
'politician' becomes 'tory' (term for a Conservative Party politician).
'ad' backwards is 'da'.
'man'+'da'+'tory'='MANDATORY'
(Other definitions for mandatory that I've seen before include "Not optional" , "required" , "Obligatory" , "allowing no option" , "Binding" .)