Gardener with top rating, one landed gentry ultimately toast (10,5)
I believe the answer is:
capability brown
'gardener' is the definition.
(famous English 18c gardener)
'top rating one landed gentry ultimately toast' is the wordplay.
'top' becomes 'cap' (both can mean to beat or outdo).
'rating' becomes 'ab' (both can mean a sailor).
'one landed gentry ultimately' becomes 'ility' (I am not sure about this - if you are sure you should believe this answer much more).
'toast' becomes 'brown' (to toast something is to brown it).
'cap'+'ab'+'ility'+'brown'='CAPABILITY BROWN'
'with' is the link.
(Other definitions for capability brown that I've seen before include "Landscape artist" , "Lancelot" , "Gardener" , "fellow good at planning growth" .)