Lower of two numbers (6)
I believe the answer is:
nether
'lower' is the definition.
(as in nether regions)
'two numbers' is the wordplay.
'two numbers' can mean two substitutions for 'number'.
'number' becomes 'n' (n can represent any number).
'number' becomes 'ether' (anaesthetic, something which numbs).
'n'+'ether'='NETHER'
'of' is the link.
(Other definitions for nether that I've seen before include "Situated below" , "Underground" , "Lower; Dutch '-lands'" , "These regions are hell, the underworld" , "Lower in position" .)