Mule of old song (3)
I believe the answer is:
sal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mule of old song</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 sal that I've seen before include "Sarah (dim.)" , "- - volatile" , "girl?" .)