Crumble away (5)
I believe the answer is:
erode
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">crumble away</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eroding is a kind of crumbling away)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for erode that I've seen before include "'Deteriorate, wear away (5)'" , "Gradually destroy" , "Eat away" , "go to pieces!" , "Slowly remove" .)