shop girl vs shop clerk

shop girl and shop clerk both are nouns.

Word	Noun	Adjective	Verb	Adverb
shop girl	Yes	No	No	No
shop clerk	Yes	No	No	No

As nouns, shop clerk is a hypernym of shop girl; that is, shop clerk is a word with a broader meaning than shop girl:

Other hypernyms of shop girl include fille, girl, miss, missy, young lady, young woman, clerk, salesclerk, shop assistant.

shop girl (noun)	shop clerk (noun)
a young female shop assistant	a salesperson in a store