Frenchman about in dull vegetation (8)
I believe the answer is:
greenery
'vegetation' is the definition.
(thesaurus)
'frenchman about in dull' is the wordplay.
'frenchman' becomes 'rene'.
'about' says the letters should be written backwards.
'in' is an insertion indicator.
'dull' becomes 'grey' (I've seen this before**).
'rene' written backwards gives 'ener'.
'ener' going into 'grey' is 'GREENERY'.
(Other definitions for greenery that I've seen before include "Living foliage or vegetation" , "Leaves etc" , "Leafy vegetation" , "colourful stuff?" , "Leaves, maybe" .)