Lesson on origins of Indus Civilization is outstanding (7)
I believe the answer is:
classic
'outstanding' is the definition.
(thesaurus)
'lesson on origins of indus civilization' is the wordplay.
'lesson' becomes 'class' (I've seen this before).
'on' means one lot of letters go next to another.
'origins of' says to take the initial letters.
The initial letters of 'indus civilization' is 'ic'.
'class'+'ic'='CLASSIC'
'is' is the link.
(Other definitions for classic that I've seen before include "Good example of its kind" , "Work of established excellence (7)" , "Artistic work of the highest excellence" , "The best of its kind" , "Of the highest order - the Derby, for example" .)