Novel Methuselah's written about himself? (5)
I believe the answer is:
money
'novel' is the definition.
(Martin Amis novel)
'methuselah's written about himself?' is the wordplay.
'methuselah's' becomes 'my' (where the crossword setter is 'Methuselah').
'written about' indicates putting letters inside.
'himself?' becomes 'one' (I can't explain this - if you can you should believe this answer much more).
'my' enclosing 'one' is 'MONEY'.
(Other definitions for money that I've seen before include "Legal tender" , "Euro for example" , "Currency; wealth" , "Lucre" , "See 7" .)