Writer for American leader on trial (6)
I believe the answer is:
proust
'writer' is the definition.
(I know that Proust is a novelist)
'for american leader on trial' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'american' becomes 'US' (United States).
'leader on' indicates taking the first letters (the letter which leads the word).
The first letter of 'trial' is 't'.
'pro'+'us'+'t'='PROUST'
(Other definitions for proust that I've seen before include "French novelist - stupor (anag)" , "Famous French novelist in stupor" , "Marcel ---, Fr. author" , "Great French writer in stupor" , "Marcel --, French novelist 1871-1922" .)