Opposition against customary practices involving Conservative (13)
I believe the answer is:
contradiction
'opposition' is the definition.
(I know that contradiction is a type of opposition)
'against customary practices involving conservative' is the wordplay.
'against' becomes 'con' (as in pros and cons, for and against).
'customary practices' becomes 'tradition' (traditions or customs).
'involving' means one lot of letters goes inside another.
'conservative' becomes 'C' (UK political abbreviation).
'tradition' enclosing 'c' is 'tradiction'.
'con'+'tradiction'='CONTRADICTION'
(Other definitions for contradiction that I've seen before include "Statement proclaiming the opposite" , "Challenge" , "disagreement" , "opposition" , "Negation" .)