Guarantee to tone down as an author? (10)
I believe the answer is:
underwrite
'guarantee' is the definition.
(I know that underwrite is a more specific form of the action guarantee)
'tone down as an author?' is the wordplay.
'tone down as' becomes 'under' (I can't explain this - if you can you should believe this answer much more).
'an author?' becomes 'write' (I've seen this before).
'under'+'write'='UNDERWRITE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for underwrite that I've seen before include "Sign" , "Guarantee financially" , "Accept liability for" .)