Habit of PPS, say, to defend Conservative (9)
I believe the answer is:
addiction
'habit' is the definition.
(drug habit)
'pps say to defend conservative' is the wordplay.
'pps say' becomes 'addition' (a PPS is an additional remark after a PS).
'to defend' is an insertion indicator.
'conservative' becomes 'C' (UK political abbreviation).
'addition' going around 'c' is 'ADDICTION'.
'of' is the link.
(Other definitions for addiction that I've seen before include "I did act on (anag.)" , "Obsession" , "The origin of dependence" , "habit that's hard to break" , "Bug one's caught" .)