Lesson confusing head girl (5)
I believe the answer is:
class
'lesson' is the definition.
(synonyms)
'confusing head girl' is the wordplay.
'head' indicates taking the first letters.
'girl' becomes 'lass' (lass can informally mean a girl).
The initial letter of 'confusing' is 'c'.
'c'+'lass'='CLASS'
(Other definitions for class that I've seen before include "Rank" , "Kind" , "Excellent" , "period" , "pupils?" .)