Form line when entering Court of Session (5)
I believe the answer is:
class
'form' is the definition.
(synonyms)
'line when entering court of session' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'when entering' indicates putting letters inside.
'court of session' becomes 'cass' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l' placed within 'cass' is 'CLASS'.
Can you help me to learn more?
(Other definitions for class that I've seen before include "Where one learns" , "Group of students" , "Rank" , "High quality" , "People having the same social or economic status" .)