Following ethical principles, A Lincoln arranged systematic knowledge (2,3,10)
I believe the answer is:
in all conscience
'following ethical principles' is the definition.
'a lincoln arranged systematic knowledge' is the wordplay.
'arranged' indicates anagramming the letters.
'systematic knowledge' becomes 'science' (I've seen this before).
'a'+'lincoln'='alincoln'
'alincoln' with letters rearranged gives 'inallcon'.
'inallcon'+'science'='IN ALL CONSCIENCE'
(Other definitions for in all conscience that I've seen before include "Certainly; by all that's fair" , "Certainly (colloq.)" .)