Politician penning extravagant material (6)
I believe the answer is:
cotton
'material' is the definition.
(I know that cotton is a type of plant fiber)
'politician penning extravagant' is the wordplay.
'politician' becomes 'con' ().
'penning' is an insertion indicator.
'extravagant' becomes 'OTT' (short for over the top).
'con' enclosing 'ott' is 'COTTON'.
(Other definitions for cotton that I've seen before include "Fabric (of natural fibre)" , "Fibre including kapok" , "Substance like fine wool" , "Creamy-white natural fibre" , "Cloth made from fine hairs of seed pod" .)