Doctor gets Conservative backing to reveal 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')
'doctor gets conservative backing' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'gets' says to put letters next to each other.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'backing' is a reversal indicator.
'tory' reversed gives 'yrot'.
'dr'+'yrot'='DRY-ROT'
'to reveal' acts as a link.
(Other definitions for dry rot that I've seen before include "Fungus affecting wood" , "Wood-destroying fungus" , "housing problem?" , "Disease of wood not well ventilated" , "Decay of badly ventilated wood" .)