Goo, muck (6)
I believe the answer is:
sludge
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">goo</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">muck</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for sludge that I've seen before include "'Muck, slime (6)'" , "Thick greasy mud" , "Ooze" , "Mire - sewage" , "Muddy sediment" .)