Catch woman on underground (6)
I believe the answer is:
nether
'underground' is the definition.
(I've seen this before)
'catch woman on' is the wordplay.
'catch' becomes 'net' (netting is a kind of catching).
'woman' becomes 'her' (**).
'on' means one lot of letters go next to another.
'net'+'her'='NETHER'
(Other definitions for nether that I've seen before include "Subordinate" , "These garments are for the bottom half" , "North European country" , "Located below" , "inferior" .)