Scholar’s leading part in standard assessment task (6)
I believe the answer is:
savant
'scholar's' is the definition.
(savant is a kind of scholar)
'leading part in standard assessment task' is the wordplay.
'leading part' becomes 'van' ('van' can mean the front of something).
'in' means one lot of letters goes inside another.
'standard assessment task' becomes 'sat' (former name for UK SAT exam).
'van' inserted within 'sat' is 'SAVANT'.
(Other definitions for savant that I've seen before include "Scholar" , "Melchior?" , "Intellectual" , "Man of great learning" , "Wise fellow" .)