Fungus beginning to spread in furrow (4)
I believe the answer is:
rust
'fungus' is the definition.
(I know that rust is a type of fungus)
'beginning to spread in furrow' is the wordplay.
'beginning to' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'furrow' becomes 'rut' (rut is a kind of furrow).
The first letter of 'spread' is 's'.
's' going within 'rut' is 'RUST'.
(Other definitions for rust that I've seen before include "Reddish-brown coating formed on iron by moisture" , "A fungus; a corrosion" , "Reddish coating formed on iron through 11 across" , "Corrode" , "Oxidised iron" .)