Detailed, thorough (7)
I believe this clue is a double definition.
'detailed' is the first definition.
(I've seen this before)
'thorough' is the second definition.
(I've seen this before)
(Other definitions for in-depth that I've seen before include "Detailed and thorough" , "Profoundly (studied)" , "Thorough and intensive" , "Comprehensive and thorough" , "Detailed (investigation)" .)