Bag lady's underground (6)
I believe the answer is:
nether
'underground' is the definition.
(I've seen this before)
'bag lady's' is the wordplay.
'bag' becomes 'net' (I've seen this in other clues).
'lady' becomes 'her' (I've seen this before**).
'net'+'her'='NETHER'
(Other definitions for nether that I've seen before include "North European country" , "Lower; Dutch '-lands'" , "These garments are for the bottom half" , "These regions are beneath" , "Subordinate" .)