Estuary flowing around river bank (8)
I believe the answer is:
treasury
'bank' is the definition.
(I've seen this before)
'estuary flowing around river' is the wordplay.
'flowing' indicates anagramming the letters.
'around' indicates putting letters inside.
'river' becomes 'r'.
'estuary' anagrammed gives 'teasury'.
'teasury' placed around 'r' is 'TREASURY'.
(Other definitions for treasury that I've seen before include "The funds of a state" , "Government department in charge of finances" , "Finance ministry" , "Its valued contents" , "Place where great valuables are stored" .)