The most common and most likely answer to this crossword clue is the 3 plus 3 letter word DRY ROT
I've seen this crossword clue in Independent
D
R
Y
R
O
T
More clues leading to the result DRY ROT
- Diagnostician perhaps right person to see about infestation
- Conservative way to reverse decay
- Droll nonsense — could it bring the house down?
- Politician needs way to reverse domestic problem
- Party member, Communistreturning, concealed decay
- Dull rubbish that could bring the house down?
- Doctor right person possibly seen about infestation
- Decay of politician with spin doctor at first
- Doctor Hill retires after onset of yucky fungal infection
- Doctor turning Conservative might bring the house down
Buddy explains!

The straight part
We know that "infestation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infestation = dry rot.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does DRY ROT mean?
DRY ROT noun- The crumbly, friable decayed portions of wooden members of buildings, especially at or below grade, usually caused by a fungal infection.
- Any progression of decay, corruption, or obsolescence.
- A fungal infection which affects plants, in particular potatoes.