Bottom two numbers (6)
I believe the answer is:
nether
'bottom' is the definition.
(similar in meaning)
'two numbers' is the wordplay.
'two numbers' means two substitutions for 'number'.
'number' becomes 'n' (n can represent any number).
'number' becomes 'ether' (anaesthetic, something which numbs).
'n'+'ether'='NETHER'
(Other definitions for nether that I've seen before include "Situated below" , "Underground" , "These regions are hell, the underworld" , "These regions are beneath" , "inferior" .)