Irregular purchase involving politician (5)
I believe the answer is:
bumpy
'irregular' is the definition.
(thesaurus)
'purchase involving politician' is the wordplay.
'purchase' becomes 'buy' (buy is a kind of purchase).
'involving' is an insertion indicator.
'politician' becomes 'MP' (Member of Parliament).
'buy' going around 'mp' is 'BUMPY'.
(Other definitions for bumpy that I've seen before include "Jolty" , "My pub (anag.)" , "Rough - turbulent" , "affected by air pockets" , "Uneven (surface); jolting" .)