One girl joined two (8)
I believe the answer is:
rosemary
'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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'girl' becomes 'Mary' (girl's name).
'rose'+'mary'='ROSEMARY'
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" .)