Subsidiary topic (4,5)
I believe the answer is:
side issue
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">subsidiary topic</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for side issue that I've seen before include "matter of lesser importance" , "Incidental minor matter" , "Illegitimate children" , "Ancillary consideration" , "That's not so important" .)