Schoolgirl in company with a boy (4)
I believe the answer is:
coed
'schoolgirl' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
'company with a boy' is the wordplay.
'company' becomes 'co' (abbreviation).
'with' means one lot of letters go next to another.
'a boy' becomes 'ed' (short for Edward).
'co'+'ed'='COED'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for coed that I've seen before include "High school pupil" , "American girl at school" , "US schoolgirl" .)