Common man takes on popular politician (8)
I believe the answer is:
minister
'politician' is the definition.
(I've seen this before)
'common man takes on popular' is the wordplay.
'common man' becomes 'mister' (standard form of address for a man).
'takes on' indicates putting letters inside.
'popular' becomes 'in' ('in' can mean trendy).
'mister' going around 'in' is 'MINISTER'.
(Other definitions for minister that I've seen before include "Agent" , "Form of address" , "Member of the clergy or of the Front Bench" , "Holder of office in government or church" , "Member of government or clergy" .)