Or else (4)
'or else' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for otherwise that I've seen before include "in different circumstances" , "White rose seen in a different way" , "White rose (anag.)" , "Apart from that" , "In other respects or ways" .)
(Other definitions for lest that I've seen before include "For fear that (something happens)" , "To avoid the risk that - we forget?" , "In case" , "For fear that; in order to prevent" , "To avoid the risk that - let's do it" .)