Volcanic rock at the bottom beginning to topple (6)
I believe the answer is:
basalt
'volcanic rock' is the definition.
(basalt is a kind of volcanic rock)
'at the bottom beginning to topple' is the wordplay.
'at the bottom' becomes 'basal' (I've seen this in another clue).
'beginning to' says to take the initial letters.
The first letter of 'topple' is 't'.
'basal'+'t'='BASALT'
(Other definitions for basalt that I've seen before include "Rock containing olivine" , "Black volcanic rock" , "Dark fine-grained volcanic rock" , "Type of rock is a blast, oddly" , "Common volcanic rock" .)