Exchanging letters (13)
I believe the answer is:
corresponding
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">exchanging letters</span>' is the definition.<br/ ><span class="explanation-format__explanation">(to correspond is to send and receive letters)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for corresponding that I've seen before include "Conforming in every respect" , "Reciprocal" .)