Canal feature; fasten (4)
I believe the answer is:
lock
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">canal feature</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">fasten</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that lock is a more specific form of the action fasten)</span><br/ ><br/ ></span>
(Other definitions for lock that I've seen before include "element in pack" , "Secure a piece of hair" , "Strand of hair" , "Get stuck in" , "Security device - bit of hair" .)