Essay - nice surprise - is English (8)
I believe the answer is:
treatise
'essay' is the definition.
(I've seen this before)
'nice surprise is english' is the wordplay.
'nice surprise' becomes 'treat' (I've seen this before).
'english' becomes 'e' (abbreviation).
'treat'+'is'+'e'='TREATISE'
(Other definitions for treatise that I've seen before include "Learned written work" , "Written composition" , "Essay on theme" , "Disquisition" , "Systematically-written formal piece" .)