Attlee was involved with reform (not half!), establishing this (7,5)
I believe the answer is:
welfare state
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'attlee was involved with reform not half' is the wordplay.
'involved with' indicates anagramming the letters (involved can mean confusing or complex).
'not half' suggests halving the letters.
'reform' with half the letters taken is 'ref'.
'attlee'+'was'='attleewas'
'attleewas' mixed up with 'ref' can make 'WELFARE STATE'.
'establishing' acts as a link.
Can you help me to learn more?
(Another definition for welfare state that I've seen is " free in our country?".)