Best garden oddly by a river (8)
I believe the answer is:
greatest
'best' is the definition.
('greatest' can be similar in meaning to 'best')
'garden oddly by a river' is the wordplay.
'oddly' indicates alternate letters.
'by' means one lot of letters go next to another.
'river' becomes 'test' (I've seen this before**).
The alternating letters of 'garden' are 'gre'.
'gre'+'a'+'test'='GREATEST'
(Other definitions for greatest that I've seen before include "Biggest and best, like Muhammad Ali" , "Most important!" , "Not to be surpassed" , "Highest in quality or in quantity" , "Get stare from the mightiest" .)