Uncertain number, number that's lower (6)
I believe the answer is:
nether
'lower' is the definition.
(as in nether regions)
'uncertain number number' is the wordplay.
'uncertain number' becomes 'n' (n can represent any number. I am not sure about the 'uncertain' bit.).
'number' becomes 'ether' (anaesthetic, something which numbs).
'n'+'ether'='NETHER'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for nether that I've seen before include "Located below" , "Situated below" , "Lower; Dutch '-lands'" , "Lower in position" , "These regions are beneath" .)