Century after century by India, restorative (5)
I believe the answer is:
tonic
'restorative' is the definition.
(I know that restorative can be written as tonic)
'century after century by india' is the wordplay.
'century' becomes 'ton' (both can mean one hundred of something).
'after' says to put letters next to each other.
'century' becomes 'C' (common abbreviation).
'by' says to put letters next to each other.
'india' becomes 'i' (phonetic alphabet: alpha, bravo, charlie etc.).
'c' put after 'i' is 'ic'.
'ton'+'ic' is 'TONIC'.
(Other definitions for tonic that I've seen before include "An enlivener" , "Important note" , "something invigorating" , "Small drink of spirits" , "do for musicians" .)