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' (informal term for a Conservative Party member).
'way' becomes 'rd' (short for road).
'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" .)