Boil gently (6)
I believe the answer is:
simmer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">boil gently</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 simmer that I've seen before include "Bubble gently just below boiling point" , "Bubble away" , "Boil slowly at low temperature" , "Bubble on a low gas" , "Gently boil" .)