Unexpectedly retain use of tidal region (9)
I believe the answer is:
estuarine
'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" .)