Gradual decline (7)
I believe the answer is:
erosion
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gradual decline</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for erosion that I've seen before include "dying away" , "Wearing away (of soil, rocks etc)" , "Mechanical process of wearing something down" , "Abrasion" , "being eaten away" .)