Relaxed group of students entering fashionable area on lake (8)
I believe the answer is:
informal
'relaxed' is the definition.
(informal can mean relaxed or casual)
'group of students entering fashionable area on lake' is the wordplay.
'group of students' becomes 'form'.
'entering fashionable area' becomes 'ina' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'lake' becomes 'L' (geographical abbreviation).
'form' inserted inside 'ina' is 'informa'.
'informa'+'l'='INFORMAL'
(Other definitions for informal that I've seen before include "Casual and unceremonious" , "Cosy -- loose" , "If normal, be casual and without ceremony" , "Unofficial, relaxed" , "Casual or unofficial" .)