Mildew (5)
I believe the answer is:
mould
'mildew' is the definition.
(I know that mildew can be written as mould)
This is the entire clue.
(Other definitions for mould that I've seen before include "Fungal growth; template" , "Must" , "Cast; form" , "Crumbly soil" , "Cause of discoloration" .)