One girl that is under another (6)
I believe the answer is:
lassie
'one girl that is under another' is the definition.
'lassie' can be an answer for 'one girl' (lassie can informally mean a girl). I'm not sure about the rest of the definition.
'one girl that is under' is the wordplay.
'one girl' becomes 'lass' (lass can informally mean a girl).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'lass'+'ie'='LASSIE'
Can you help me to learn more?
(Other definitions for lassie that I've seen before include "dog in pictures" , "Hollywood film-star dog" , "Screen canine" , "Movie role for several dog stars" , "''Will ye go ..., go'' (folksong)" .)