Monotonous hill -- turned back, as something dangerous in wood (3,3)
I believe the answer is:
dry rot
'something dangerous in wood' is the definition.
(type of decay in wood)
'monotonous hill turned back' is the wordplay.
'monotonous' becomes 'dry'.
'hill' becomes 'tor' (tor is a kind of hill).
'turned back' shows that the letters should be reversed in order.
'tor' reversed gives 'rot'.
'dry'+'rot'='DRY-ROT'
'as' acts as a link.
(Other definitions for dry rot that I've seen before include "Fungus attacking wood" , "Disease of wood not well ventilated" , "domestic problem" , "it may make one's beam disappear" , "problem with deal?" .)