Crossword Genius

What seasonally encourages a romantic understanding? (9)

Ross

I believe the answer is:

mistletoe

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

'understanding?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for mistletoe that I've seen before include "Kissed-under plant" , "it may get you a few smackers" , "Christmas berry" , "Parasitic shrub for Christmas" , "incitement to lovers?" .)

I've seen this clue in The Times.
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