Conservative way to reverse decay (3,3)
I believe the answer is:
dry rot
'decay' is the definition.
(I have seen 'Decay of timber ' mean 'dry rot' so perhaps 'decay' could also mean 'dry rot')
'conservative way to reverse' is the wordplay.
'conservative' becomes 'tory' (I've seen this before).
'way' becomes 'rd'.
'to reverse' shows that the letters should be reversed in order.
'tory'+'rd'='toryrd'
'toryrd' backwards is 'DRY-ROT'.
(Other definitions for dry rot that I've seen before include "Fungus that causes wood to crumble" , "Decay of unventilated wood" , "infestation" , "problem with truss?" , "Fungus affecting wood" .)