Cook gently (6)
I believe the answer is:
simmer
'cook gently' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for simmer that I've seen before include "Cook in gently boiling water" , "Cook just below boiling point" , "Bubble on a low gas" , "Bubble away" , "Bubble gently just below boiling point" .)