The last word about sources of Cambridge undergrads’ discernment (6)
I believe the answer is:
acumen
'discernment' is the definition.
(thesaurus)
'the last word about sources of cambridge undergrads'' is the wordplay.
'the last word' becomes 'amen' (I've seen this before).
'about' is an insertion indicator.
'sources of' suggests taking the first letters.
The first letters of 'cambridge undergrads' is 'cu'.
'amen' going around 'cu' is 'ACUMEN'.
(Other definitions for acumen that I've seen before include "Ability to take well judged decisions" , "Forming judgement" , "Penetrating insight" , "Sharpness, penetration, quickness of perception" , "Intellectual sharpness as in business" .)