Grow by addition (6)
I believe the answer is:
accrue
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grow by addition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for accrue that I've seen before include "Be added to, develop" , "Accumulate over time" , "(Of money) build up in regular amounts" , "Grow like interest on money" , "Build up interest" .)