Crossword Genius

Unexpectedly retain use of tidal region (9)

Ross

I believe the answer is:

estuarine

I'm a little stuck... Click here to teach me more about this clue!

'unexpectedly retain use of tidal region' is the definition.
I can't tell whether this defines the answer.

'unexpectedly retain use' is the wordplay.
'unexpectedly' indicates an anagram.
'retain'+'use'='retainuse'
'retainuse' anagrammed gives 'ESTUARINE'.

Can you help me to learn more?

(Other definitions for estuarine that I've seen before include "of Essex, perhaps" , "In part of the river" , "mouth" .)

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