Pile up (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pile up</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for amass that I've seen before include "run up?" , "Accrue, heap up" , "Get together" , "Collect, hoard" , "store" .)