Decay of timber (3,3)
I believe the answer is:
dry rot
'decay of timber' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dry rot that I've seen before include "Decay of wood poorly ventilated" , "Decay of wood not well ventilated" , "Decay of badly ventilated wood" , "domestic problem" , "Wood-destroying fungus" .)