Girl sat behind Charlie in form (5)
I believe the answer is:
class
'form' is the definition.
(I know that form can be written as class)
'girl sat behind charlie' is the wordplay.
'girl sat' becomes 'cl' (I can't justify this - if you can you should believe this answer much more).
'behind' means one lot of letters go next to another.
'charlie' becomes 'ass' (both can mean a foolish person).
'cl'+'ass'='CLASS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for class that I've seen before include "Stylish; lesson" , "High-quality" , "Genus" , "Fine" , "sort" .)