One girl joined two (8)
I believe the answer is:
'joined two' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one girl' is the wordplay.
'one' becomes 'rose' (I can't explain this - if you can you should believe this answer much more).
'girl' becomes 'Mary' (girl's name).
Can you help me to learn more?
(Other definitions for rosemary that I've seen before include "Herb - girl's name(s)" , "Fragrant evergreen shrub used in cooking" , "Name of girl or herb" , "such seasons" , "Evergreen flowering shrub - girl's name" .)