Remain valid for Brown following conclusion of discussions with Unionists (5,2)
I believe the answer is:
stand up
'remain valid' is the definition.
(thesaurus)
'brown following conclusion of discussions with unionists' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown**).
'following' says to put letters next to each other.
'conclusion of' says to take the final letters.
'with' means one lot of letters go next to another.
'unionists' becomes 'dup' (Democratic Unionist Party).
The last letter of 'discussions' is 's'.
'tan' after 's' is 'stan'.
'stan'+'dup'='STAND-UP'
'for' acts as a link.
(Other definitions for stand up that I've seen before include "acceptable" , "Variety of comedy" , "Performed solo (of a comedy routine)" , "Type of comedian" , "Arise, Dame Edna!" .)