Politician on road 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')
'politician on road to reverse' is the wordplay.
'politician' becomes 'tory' (term for a Conservative Party politician).
'on' says to put letters next to each other.
'road' becomes 'rd' (abbreviation).
'to reverse' says the letters should be written in reverse.
'tory'+'rd'='toryrd'
'toryrd' in reverse letter order is 'DRY-ROT'.
(Other definitions for dry rot that I've seen before include "corruption" , "Decay of wood poorly ventilated" , "Decay of badly ventilated wood" , "Wood-destroying fungus" , "fungal infection" .)