Timber decay (3,3)
I believe the answer is:
dry rot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">timber decay</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)