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' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'doctor' becomes 'dr' (abbreviation).
'tory' backwards 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" .)