Something from Sturgeon, good girl after one transgression (9)
I believe the answer is:
isinglass
'something from sturgeon' is the definition.
I can't judge whether this defines the answer.
'good girl after one transgression' is the wordplay.
'good' becomes 'g' (abbreviation).
'girl' becomes 'lass' (lass can informally mean a girl).
'after' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'transgression' becomes 'sin' (sin is a kind of transgression).
'g'+'lass'='glass'
'i'+'sin'='isin'
'glass' after 'isin' is 'ISINGLASS'.
Can you help me to learn more?
(Other definitions for isinglass that I've seen before include "What's got from fish" , "Some Muscovite" , "Sis, a sling turns out to be a gelatin" , "Fish gelatin" , "Gelatine" .)