Mimosa ultimately is a shrub raised in a large part of the world (7)
I believe the answer is:
eurasia
'part of the world' is the definition.
(I've seen this before)
'mimosa ultimately is a shrub raised in a' is the wordplay.
'ultimately' suggests the final letters.
'a shrub' becomes 'rue' (I am not sure about this - if you are sure you should believe this answer much more).
'raised' says the letters should be written in reverse (going up in a down clue).
'in' is an insertion indicator.
The last letter of 'mimosa' is 'a'.
'is'+'rue'='isrue'
'isrue' in reverse letter order is 'eursi'.
'a' going inside 'eursi' is 'eurasi'.
'eurasi'+'a'='EURASIA'
'large' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for eurasia that I've seen before include "combination of continents" , "Landmass extending west-east from Cape Roca to Cape Dezhnev" , "large part of world" , "large bloc of countries" , "Largest landmass" .)