Stand to reason with politicians after publicity (3,2)
I believe the answer is:
add up
'stand to reason' is the definition.
(I've seen this in another clue)
'politicians after publicity' is the wordplay.
'politicians' becomes 'DUP' (Democratic Unionist Party).
'after' means one lot of letters go next to another.
'publicity' becomes 'ad' (short for advertisement).
'dup' after 'ad' is 'ADD UP'.
'with' is the link.
(Other definitions for add up that I've seen before include "Aggregate, get total" , "Do sums" , "Sum - make sense" , "Seem to be sensible" , "Make sense; count" .)