It's right for a river to be bucolic (5)
I believe the answer is:
rural
'be bucolic' is the definition.
(similar in meaning)
'it's right for a river' is the wordplay.
'it's right' indicates one should take the final letters (I've seen 'right of' mean this (letter on the rightmost end of the word)).
'a river' becomes 'ural' (Russian river).
The last letter of 'for' is 'r'.
'r'+'ural'='RURAL'
'to' is the link.
(Other definitions for rural that I've seen before include "living in the countryside" , "Connected with farming" , "Sort of dean" , "Of the country - pastoral" , "Characteristic of country life" .)