We must hold girl back at first, potential starlet (7)
I believe the answer is:
wannabe
'potential starlet' is the definition.
'we must hold girl back at first' is the wordplay.
'must hold' indicates putting letters inside.
'girl' becomes 'anna' ().
'at first' suggests taking the first letters.
The initial letter of 'back' is 'b'.
'anna'+'b'='annab'
'we' placed around 'annab' is 'WANNABE'.
(Other definitions for wannabe that I've seen before include "One desiring to resemble someone else" , "hopeful" , "Ambitious person" , "Aspirant" , "Aspiring film star possibly" .)