Crazy, in Cancún (4)
I believe the answer is:
loco
'crazy in cancun' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for loco that I've seen before include "Engine - nuts" , "Barmy (Spanish) - engine (abbr)" , "Transport" , "plant increasing milk yield" , "Crazy; engine" .)