Tarnish articulate BBC politics presenter (3)
I believe the answer is:
mar
'tarnish' is the definition.
(thesaurus)
'articulate bbc politics presenter' is the wordplay.
'articulate' shows a homophone (sound like) (I've seen 'articulated' mean this).
'bbc politics presenter' becomes 'marr' (broadcaster Andrew Marr).
'marr' sounds like 'MAR'.
(Other definitions for mar that I've seen before include "Oh dear!" , "Detract from" , "Disfigure" , "Spanish main?" , "Spoil stuff" .)