Fungal disease coats small English apple (6)
I believe the answer is:
russet
'apple' is the definition.
(a russet-coloured apple)
'fungal disease coats small english' is the wordplay.
'fungal disease' becomes 'rust' (rust is a kind of disease. I am not sure about the 'fungal' bit.).
'coats' indicates putting letters inside (inserted letters are coated in another word).
'small' becomes 's' (abbreviation - e.g. clothes size).
'english' becomes 'E' (abbreviation).
's'+'e'='se'
'rust' going around 'se' is 'RUSSET'.
Can you help me to learn more?
(Other definitions for russet that I've seen before include "Apple - colour" , "Reddish-brown - not orange!" , "Autumnal reddish-brown colour" , "Reddish brown; apple" , "Surest type of colour" .)