Fertilise the garden finally with flush (6)
I believe the answer is:
enrich
'fertilise' is the definition.
(thesaurus)
'the garden finally with flush' is the wordplay.
'finally' says to take the final letters.
'with' means one lot of letters go next to another.
'flush' becomes 'rich' (similar in meaning).
The last letters of 'the garden' are 'en'.
'en'+'rich'='ENRICH'
(Other definitions for enrich that I've seen before include "Make fuller flavoured" , "Adorn, enhance or make wealthy" , "Concentrate" , "Heighten" , "increase wealth" .)