… in Chelsea or otherwise (4)
I believe the answer is:
else
'or otherwise' is the definition.
(I've seen this before)
'in chelsea' is the wordplay.
'in' says the answer is hidden in the clue.
'ELSE' can be found hidden inside 'chelsea'.
(Other definitions for else that I've seen before include "Besides; instead" , "Alternatively" , "Otherwise; different from" , "In addition; different" , "In addition, besides" .)