Africa's longest river (4)
I believe the answer is:
nile
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">africa's longest river</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for nile that I've seen before include "'The longest river, blue and white (4)'" , "Long river" , "African river" , "Egyptian river" , "River that's Blue and White" .)