Crossword Genius

Preoccupation in gangland brothers, it's said (5)

Ross

I believe the answer is:

craze

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'preoccupation' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for craze that I've seen before include "Fine crack" , "passing fashion" , "Mania" , "passion" , "latest thing" .)

I've seen this clue in The Independent.
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