Part of the wide estuary of a river (3)
I believe the answer is:
dee
'a river' is the definition.
(River Dee)
'part of the wide estuary of' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'DEE' is hidden within 'wide estuary of'.
(Other definitions for dee that I've seen before include "River name" , "Scottish river" , "River of Scotland" , "John ---, Elizabethan mage" , "Name of many rivers" .)