Timber decay (3,3)
I believe the answer is:
dry rot
'timber decay' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dry rot that I've seen before include "infestation" , "problem with deal?" , "Decay of wood poorly ventilated" , "Decay of wood not well ventilated" , "it may make one's beam disappear" .)