Online engine? That's crazy! (4)
I believe the answer is:
loco
This clue could be a double definition.
'online engine?' is the first definition.
'loco' can be an answer for 'engine?' (I've seen this before). I am unsure of the 'online' bit.
'crazy' is the second definition.
(loco can informally mean mad)
'that's' is the link.
Can you help me to learn more?
(Other definitions for loco that I've seen before include "It's used for drawing" , "Barmy (Spanish) - engine (abbr)" , "In ... parentis, in place of parents" , "Mad - train" , "Crazy in America - cool, man" .)