Form C girl (5)

I believe the answer is:

class

'form' is the definition.
(I know that form can be written as class)

'c girl' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'c'+'lass'='CLASS'

(Other definitions for class that I've seen before include "School group" , "High-quality" , "showing stylish excellence" , "Order - quality" , "Group - style" .)

I've seen this clue in The Telegraph.

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