Horse carrying Frenchman northward leaves, say? (8)
I believe the answer is:
greenery
'leaves say?' is the definition.
(green leaves)
'horse carrying frenchman northward' is the wordplay.
'horse' becomes 'grey' (I've seen this before).
'carrying' means one lot of letters goes inside another.
'frenchman' becomes 'rene' (common French name).
'northward' says the letters should be written backwards (in down clue going up).
'rene' reversed gives 'ener'.
'grey' placed around 'ener' is 'GREENERY'.
(Other definitions for greenery that I've seen before include "Trees" , "Living foliage or vegetation" , "Colourful display" , "Decorative boughs" , "Foliage etc" .)