Strange mysteries enshroud Clement Attlee's origins, you must agree (1,4,2,4)
I believe the answer is:
i rest my case
'you must agree' is the definition.
(statement said after presenting an argument)
'strange mysteries enshroud clement attlee's origins' is the wordplay.
'strange' is an anagram indicator.
'enshroud' is an insertion indicator.
'origins' indicates taking the first letters.
The first letters of 'clement attlees' is 'ca'.
'mysteries' anagrammed gives 'irestmyse'.
'irestmyse' going around 'ca' is 'I REST MY CASE'.
(Another definition for i rest my case that I've seen is " That's all I need to say".)