Unfinished book, old one, right next to tank (9)
I believe the answer is:
reservoir
'to tank' is the definition.
(reservoir is a kind of tank)
'unfinished book old one right' is the wordplay.
'unfinished' means to remove the last letter.
'book' becomes 're-serve' (as in booking or reserving in advance).
'old one right' becomes 'oir' (I can't explain this - if you can you should believe this answer much more).
'reserve' with its last letter taken off is 'reserv'.
'reserv'+'oir'='RESERVOIR'
'next' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for reservoir that I've seen before include "Store for water etc" , "depot" , "Man-made lake of stored water" , "Man-made lake to store water" , "River rose (anag.)" .)