Nuts or crackers (4)
I believe the answer is:
loco
I believe this clue is a double definition.
'nuts' is the first definition.
(synonyms)
'crackers' is the second definition.
(synonyms)
(Other definitions for loco that I've seen before include "Engine (abbrev.)" , "Engine - nuts" , "plant increasing milk yield" , "Potty" , "Barmy (Spanish) - engine (abbr)" .)