New anaesthetic for bottom (6)
I believe the answer is:
nether
'bottom' is the definition.
('nether' can be similar in meaning to 'bottom')
'new anaesthetic' is the wordplay.
'new' becomes 'n'.
'anaesthetic' becomes 'ether' (ether is a kind of anaesthetic).
'n'+'ether'='NETHER'
'for' acts as a link.
(Other definitions for nether that I've seen before include "Located below" , "Subordinate" , "North European country" , "Underground" , "inferior" .)