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' (informal term for a Conservative Party member).
'upset' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'doctor' becomes 'dr' (abbreviation).
'tory' reversed gives 'yrot'.
'yrot' put 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" .)