To support Conservative in boast is wrong as a reassertion of standards (4,2,6)
I believe the answer is:
back to basics
'boast is wrong as a reassertion of standards' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'to support conservative in' is the wordplay.
'support' becomes 'back' (back is a kind of support).
'conservative' becomes 'basics' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'back'+'basics'='backbasics'
'to' put into 'backbasics' is 'BACK TO BASICS'.
Can you help me to learn more?
(Another definition for back to basics that I've seen is " John Major's political slogan".)