Threaten one politician with ruin? (6)
I believe the answer is:
impend
'threaten' is the definition.
(I've seen this before)
'one politician with ruin?' is the wordplay.
'one politician' becomes 'imp' (I've seen this in other clues).
'with' means one lot of letters go next to another.
'ruin?' becomes 'end' (ruin something means to end it).
'imp'+'end'='IMPEND'
(Other definitions for impend that I've seen before include "Threaten; approach" , "Be imminent or about to happen" , "appearing to be imminent" , "Be about to take place" , "Become imminent" .)