Sparkling river entering bay (6)
I believe the answer is:
bright
'sparkling' is the definition.
(similar in meaning)
'river entering bay' is the wordplay.
'river' becomes 'r' (abbreviation).
'entering' is an insertion indicator.
'bay' becomes 'bight' (bight is a kind of bay).
'r' inserted into 'bight' is 'BRIGHT'.
(Other definitions for bright that I've seen before include "Radiant or intelligent" , "Illuminated" , "Clear" , "Vivid - promising - smart" , "Shining (with intelligence)" .)