Fasten, affix (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 know that fasten is a more specific form of the action attach)</span><br/ ><br/ >'<span class="explanation-format__original">affix</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that affix is a more specific form of the action attach)</span><br/ ><br/ ></span>
(Other definitions for attach that I've seen before include "Document carriers" , "Add on to, couple" , "Unite?" , "Join or seize by law" , "'Affix, fasten (6)'" .)