Crossword Genius

Crazy, yet possibly having a motive (4)

Ross

I believe the answer is:

loco

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'crazy' is the definition.
(loco can informally mean mad)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for loco that I've seen before include "plant increasing milk yield" , "Potty" , "Mad - train" , "In ... parentis, in place of parents" , "Transport" .)

I've seen this clue in the Evening Standard.
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