Russian capital conceals last of bomb debris (6)
I believe the answer is:
rubble
'debris' is the definition.
(synonyms)
'russian capital conceals last of bomb' is the wordplay.
'russian capital' becomes 'ruble' (I've seen this in another clue).
'conceals' means one lot of letters goes inside another.
'last of' suggests the final letters.
The final letter of 'bomb' is 'b'.
'ruble' going around 'b' is 'RUBBLE'.
(Other definitions for rubble that I've seen before include "Remains of demolished building" , "Waste fragments of stone or rock, debris" , "Mass of rough fragments of brick or stone" , "Bits of broken stone, brick etc" , "Stone/brick waste" .)