Drop English to a large extent (5)
I believe the answer is:
quite
'to a large extent' is the definition.
(I've seen this in another clue)
'drop english' is the wordplay.
'drop' becomes 'quit' (I've seen this in another clue).
'english' becomes 'E' (abbreviation as in OED).
'quit'+'e'='QUITE'
(Other definitions for quite that I've seen before include "Somewhat; fully" , "Rather" , "Somewhat; completely" , "To a degree" , "Fairly; indeed" .)