The most common and most likely answer to this crossword clue is the 9 letter word INCUBATES
I've seen this crossword clue in The Times
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More clues leading to the result INCUBATES
Buddy explains!

The straight part
We know that "broods" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case broods = incubates.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does INCUBATES mean?
INCUBATES verb- To brood, raise, or maintain eggs, organisms, or living tissue through the provision of ideal environmental conditions.
- To incubate metaphorically; to ponder an idea slowly and deliberately as if in preparation for hatching it.