River in India, certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'river in india' is the wordplay.
'river' becomes 'dee' (River Dee).
'in' means one lot of letters goes inside another.
'india' becomes 'ind' (abbreviation).
'dee' placed inside 'ind' is 'INDEED'.
(Other definitions for indeed that I've seen before include "Without question" , "Truthfully, really" , "In fact, really" , "Really; certainly" , "Unquestionably" .)