Crazy, yet possibly having a motive (4)
I believe the answer is:
loco
'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" .)