Highly developed knowledge about English (4)
I believe the answer is:
keen
'highly developed' is the definition.
(eg a keen sense of smell)
'knowledge about english' is the wordplay.
'knowledge' becomes 'ken' (ken is a kind of knowledge).
'about' indicates putting letters inside.
'english' becomes 'E' (abbreviation as in OED).
'ken' going around 'e' is 'KEEN'.
(Other definitions for keen that I've seen before include "Agog" , "devoted" , "mourn for the departed" , "Eager - to lament the dead" , "Enthusiastic or sharp" .)