French article about one million trees (5)
I believe the answer is:
limes
'trees' is the definition.
(lime is a kind of tree)
'french article about one million' is the wordplay.
'french article' becomes 'les' (plural definite article in French).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'million' becomes 'm' (abbreviation).
'i'+'m'='im'
'les' enclosing 'im' is 'LIMES'.
(Other definitions for limes that I've seen before include "Green fruit" , "Green fruits" , "Citrus fruit" , "Citrus trees" , "Citrus fruits" .)