Type of festive wine drunk without hesitation (6)

I believe the answer is:

mulled

I'm a little stuck... Click here to teach me more about this clue!

'drunk without hesitation' is the definition.
Although both the answer and definition are past participle verbs, I cannot see how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mulled that I've seen before include "Pondered" , "Warm, spiced (wine)" , "Of wine, heated with sugar and spices" .)

Want a hint initially instead of a full solution? Install my app