Lil, one of a girl's best friends! (7)
I believe the answer is:
diamond
'best friends' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'lil one of a girl's' is the wordplay.
I cannot really understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'a' is found in the answer.
'girl' could be 'di' and 'di' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "(Usually colourless) gemstone - card" , "Small type" , "can be rough" , "Very hard substance" , "American pitch" .)