Almost boil when new premium's not taken up (6)
I believe the answer is:
simmer
'almost boil' is the definition.
'simmer' can be an answer for 'boil' (simmering is a kind of boiling). I am not certain of the 'almost' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for simmer that I've seen before include "Gently boil" , "Cook in gently boiling water" , "Cook just below boiling point" , "Boil slowly at low temperature" , "Bubble away" .)