Study the echo (10)
I believe the answer is:
reflection
I believe this clue is a double definition.
'study' is the first definition.
(thesaurus)
'echo' is the second definition.
(thesaurus)
'the' is the link.
(Other definitions for reflection that I've seen before include "Effect of not absorbing light" , "Mirror image" , "serious thought?" , "mental study's required" .)