Conservative upset by doctor's 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 upset by doctor's' is the wordplay.
'conservative' becomes 'tory' (Tory means a member of the Conservative Party).
'upset' is a reversal indicator.
'by' says to put letters next to each other.
'doctor' becomes 'dr' (abbreviation).
'tory' back-to-front is 'yrot'.
'yrot' after 'dr' is 'DRY-ROT'.
(Other definitions for dry rot that I've seen before include "Fungus affecting wood" , "Timber disease" , "infestation" , "it may make one's beam disappear" , "Decay of wood poorly ventilated" .)