Girl having endless success getting in with US lawyer (7)
I believe the answer is:
lucinda
'in with us lawyer' is the definition.
I can't tell whether this definition defines the answer.
'girl having endless success' is the wordplay.
'girl' becomes 'linda' ().
'having' means one lot of letters goes inside another.
'endless' indicates the central letters.
'success' becomes 'luck'.
The central letters of 'luck' are 'uc'.
'linda' placed around 'uc' is 'LUCINDA'.
'getting' acts as a link.
(Other definitions for lucinda that I've seen before include "Girl" , "Woman" , "- - Lambton, architectural historian" , "Girl's name" .)