Good girl tumbler? (5)
I believe the answer is:
glass
'tumbler?' is the definition.
(tumbler is a kind of glass)
'good girl' is the wordplay.
'good' becomes 'g' (abbreviation).
'girl' becomes 'lass' (lass can informally mean a girl).
'g'+'lass'='GLASS'
(Other definitions for glass that I've seen before include "Transparent solid" , "Greenhouse material" , "Transparent substance" , "such as one might hold at party" , "Goblet" .)