Perform a favour for (6)
I believe the answer is:
oblige
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">perform a favour for</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 oblige that I've seen before include "serve" , "press!" , "Please" , "Do favour or coerce" , "Coerce or accommodate" .)