Man joins the French plant (7)
I believe the answer is:
nigella
'plant' is the definition.
(nigella is a kind of plant)
'man joins the french' is the wordplay.
'man' becomes 'nigel' ().
'joins' means one lot of letters go next to another.
'the french' becomes 'la' ('the' in French).
'nigel'+'la'='NIGELLA'
(Other definitions for nigella that I've seen before include "girl" , "Woman" , "TV personality" , "Flower found in legal" , "plant" .)