For a start, looks like new au pair's out, one being childless (9)
I believe the answer is:
nullipara
'one being childless' is the definition.
(term for a female that hasn't given birth)
'for a start looks like new au pair's out' is the wordplay.
'for a start' suggests taking the first letters.
'out' indicates an anagram (out can mean wrong or inaccurate).
The first letters of 'looks like new' is 'lln'.
'lln'+'au'+'pair'='llnaupair'
'llnaupair' is an anagram of 'NULLIPARA'.
(Other definitions for nullipara that I've seen before include "Female not having given birth" , "Woman, never giving birth" .)