Thus might timber reserve a journal (7)
I believe the answer is:
logbook
'journal' is the definition.
(thesaurus)
'thus might timber reserve a' is the wordplay.
I cannot quite see how this works, but
'reserve' could be 'book' (to book something is to reserve it) and 'book' is present in the answer.
The remaining letters 'log' is a valid word 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 logbook that I've seen before include "Former vehicle document" , "Registration paper" , "Ship's record" , "Where details of voyages are written" , "Incident-recording volume" .)