Letter-writing friends (3-4)
I believe the answer is:
pen pals
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">letter-writing friends</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for pen pals that I've seen before include "Friends who communicate in writing" , "'Friends through correspondence (3,4)'" , "Young correspondents" , "They correspond" .)