Longest UK river (6)
I believe the answer is:
severn
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">longest uk river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for severn that I've seen before include "West Country river" , "Longest British river" , "River rising in Wales" , "River flowing into the Bristol Channel" , "Major UK river" .)