New number underground (6)
I believe the answer is:
nether
'underground' is the definition.
(I've seen this before)
'new number' is the wordplay.
'new' becomes 'n'.
'number' becomes 'ether' (I've seen this before**).
'n'+'ether'='NETHER'
(Other definitions for nether that I've seen before include "These garments are for the bottom half" , "Underground" , "These regions are beneath" , "inferior" , "Located below" .)