Marines protecting India’s border (3)
I believe the answer is:
rim
'border' is the definition.
(both can mean an edge)
'marines protecting india's' is the wordplay.
'marines' becomes 'RM' (abbreviation for 'Royal Marines').
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'india' becomes 'i' (phonetic alphabet: alpha, bravo, charlie etc.).
'rm' enclosing 'i' is 'RIM'.
(Other definitions for rim that I've seen before include "'Brim, lip (3)'" , "Outer edge of a wheel" , "Brink" , "Raised edge of a round object" , "An edge, canyon or cup" .)