Do one's bit (4,2)
I believe the answer is:
muck in
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">do one's bit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for muck in that I've seen before include "Lend a hand" , "In slang, share tasks equally - even filthy ones" , "Help out" , "Do one's bit" .)