Extended treatise penned by professional wordsmith (10)
I believe the answer is:
protracted
'extended' is the definition.
(I know that extended can be written as protracted)
'treatise penned by professional wordsmith' is the wordplay.
'treatise' becomes 'tract' (tract is a kind of treatise).
'penned by' indicates putting letters inside (inserted letters are penned in).
'professional' becomes 'pro' (abbreviation).
'wordsmith' becomes 'ed' (I am not sure about this - if you are sure you should believe this answer much more).
'pro'+'ed'='proed'
'tract' inserted inside 'proed' is 'PROTRACTED'.
Can you help me to learn more?
(Other definitions for protracted that I've seen before include "Dragged out" , "Drawn-out" , "Lengthy" , "Lengthened out" .)