Irish politician crosses tiny border river
I believe the answer is:
tweed
'border river' is the definition.
(I've seen this before)
'irish politician crosses tiny' is the wordplay.
'irish politician' becomes 'td' (Teachta Dala, meaning an Irish parliament member).
'crosses' means one lot of letters goes inside another.
'tiny' becomes 'wee' (I've seen this in another clue).
'td' going around 'wee' is 'TWEED'.
(Other definitions for tweed that I've seen before include "Woollen cloth - Scotland/England border river" , "Running water" , "River on border between England and Scotland" , "passes between English and Scottish banks" , "Material; river" .)