Fasten, link (6)
I believe the answer is:
attach
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fasten</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">link</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(attaching is a kind of linking)</span><br/ ><br/ ></span>
(Other definitions for attach that I've seen before include "Tie, bind to" , "Join; seize (legal)" , "Secure" , "Join to something else" , "Tag on" .)