Accumulate over time (5)
I believe the answer is:
amass
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">accumulate over time</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 amass that I've seen before include "Accumulate, pile up" , "Collect or gather" , "Accrue, heap up" , "Gather together, pile up" , "run up?" .)