Join flower girl on top (6)
I believe the answer is:
annexe
'join' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'flower girl on top' is the wordplay.
'flower' becomes 'exe' (Exe is an English river).
'girl' becomes 'ann' (woman's name).
'on top' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'exe' put after 'ann' is 'ANNEXE'.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Adjoining building" , "Additional room etc for house" , "An addition to a document or building" , "Building attached to a largerone" , "Side building" .)