Extensive water from river in heroic tales (small unopened book) (8,3)
I believe the answer is:
sargasso sea
'unopened book' is the definition.
The definition suggests a singular noun which matches the answer.
'extensive water from river in heroic tales small' is the wordplay.
I cannot quite see how this works, but
'water' could be 'ea' (I've seen this in other clues) and 'ea' is present in the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'river' could be 'r' (abbreviation for river) and 'r' is located in the answer.
'tales' could be 'sagas' (I've seen this in another clue) and 'sagas' is found within the leftover letters.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sargasso sea that I've seen before include "Part of N. Atlantic where weed gathers" , "Weedy part of Atlantic Ocean" , "Weed-covered part of Atlantic Ocean" , "where one may be becalmed" , "weed-filled waters" .)