Very good ultimately dry area's leguminous plant (4)
I believe the answer is:
soya
'leguminous plant' is the definition.
(I know that soya is a type of leguminous plant)
'very good ultimately dry area's' is the wordplay.
'very good' becomes 'so' (as in the adjectival form 'just so!').
'ultimately' indicates one should take the final letters.
The final letters of 'dry area' are 'ya'.
'so'+'ya'='SOYA'
(Other definitions for soya that I've seen before include "Plant seed, meat substitute" , "Vegetable with very high protein" , "Dark salty sauce" , "source of protein" , "Protein-rich bean" .)