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 a hidden word.
'CRETE' is hidden within '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" .)