Lower profit by the girl (6)
I believe the answer is:
nether
'lower' is the definition.
(as in nether regions)
'profit by the girl' is the wordplay.
'profit' becomes 'net' (netting is a kind of profiting).
'by' says to put letters next to each other.
'the girl' becomes 'her' ('her' refers to a girl or woman).
'net'+'her'='NETHER'
(Other definitions for nether that I've seen before include "Lower, like regions or garments" , "These garments are for the bottom half" , "Lower; Dutch '-lands'" , "Underground" , "Located below" .)