Donkey pin-on (4)
I believe the answer is:
tail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">donkey pin-on</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tail that I've seen before include "Caudal fin" , "bum" , "Follow - rear part of aeroplane" , "Follow -- rear end" , "Last, rear, part" .)