Do something as a favour (6)
I believe the answer is:
oblige
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">do something as a favour</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 oblige that I've seen before include "Perform a service for (someone)" , "Hold in debt" , "Perform a favour for" , "Coerce or accommodate" , "Do something as a favour to (someone)" .)