Boil gently (6)
I believe the answer is:
simmer
'boil gently' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)