Island in a secret estuary (5)
I believe the answer is:
crete
'island' is the definition.
(Greek island)
'in a secret estuary' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CRETE' can be found hidden inside 'secret estuary'.
(Other definitions for crete that I've seen before include "Large Greek island" , "Island whose largest city is Heraklion" , "Major island south of Greece" , "European location" , "Island Germans took, 1941" .)