Firm new figure, take in as a mistress (9)
I believe the answer is:
concubine
'a mistress' is the definition.
(concubine is a kind of mistress)
'firm new figure take in' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'figure' becomes 'cube' (I have seen 'Solid figure' mean 'cube' so perhaps 'figure' could also mean 'cube').
'take' is an insertion indicator.
'cube' going around 'in' is 'cubine'.
'co'+'n'+'cubine'='CONCUBINE'
'as' is the link.
(Another definition for concubine that I've seen is " Unmarried woman cohabiting with a man - secondary wife".)