To burn slowly with smoke but without flames. (Cambridge Dictionary)
Burn slowly with smoke but no flame. (Oxford Dictionaries)
Show or feel barely suppressed anger, hatred, or another powerful emotion. (Oxford Dictionaries)
Exist in a suppressed or concealed state. (Oxford Dictionaries)
If something smoulders, it burns slowly, producing smoke but not flames. (Collins Dictionary)
Let not time smoulder,let it burn.