Haydn's sobriquet (4)
I believe the answer is:
papa
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">haydn's sobriquet</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 papa that I've seen before include "Pop" , "Comms code for 'P'" , "one after Oscar" , "The old man" , "Father; letter P" .)