Decay of politician with spin doctor at first (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 with spin doctor at first' is the wordplay.
'politician' becomes 'tory' (I've seen this before).
'with spin' is a reversal indicator.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'at first' says to put letters next to each other.
'tory' back-to-front is 'yrot'.
'yrot' after 'dr' is 'DRY-ROT'.
'of' acts as a link.
(Other definitions for dry rot that I've seen before include "Decay of wood poorly ventilated" , "Fungus attacking wood" , "Fungus that causes wood to crumble" , "Degeneration" , "Decay of timber caused by fungus" .)