Nice surprise found on one's English paper (8)
I believe the answer is:
treatise
'paper' is the definition.
(thesaurus)
'nice surprise found on one's english' is the wordplay.
'nice surprise' becomes 'treat' (I've seen this before).
'found on' is a charade indicator (letters next to each other).
'one's' becomes 'is' (roman numeral).
'english' becomes 'e' (abbreviation).
'treat'+'is'+'e'='TREATISE'
(Other definitions for treatise that I've seen before include "Formal written work dealing with a particular subject" , "Essay on theme" , "Written work dealing with one subject" , "Learned work formally dealing with a subject" , "A written work about treaties" .)