Formal written work — pleasant surprise is estimation originally (8)
I believe the answer is:
treatise
'formal written work' is the definition.
(I've seen this before)
'pleasant surprise is estimation originally' is the wordplay.
'pleasant surprise' becomes 'treat' (a treat is a pleasant gift or favour).
'originally' says to take the initial letters.
The first letter of 'estimation' is 'e'.
'treat'+'is'+'e'='TREATISE'
(Other definitions for treatise that I've seen before include "Learned writing" , "Academic composition" , "Written work dealing with one subject" , "Essay on theme" , "Iterates in dissertation" .)