UK’s longest river (6)
I believe the answer is:
severn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">uk's longest river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for severn that I've seen before include "Shrewsbury's river" , "GB's longest river" , "Flower" , "A major British river" , "Running water" .)