Stiff group of students supported by a lecturer (6)
I believe the answer is:
formal
'stiff' is the definition.
('formal' can be similar in meaning to 'stiff')
'group of students supported by a lecturer' is the wordplay.
'group of students' becomes 'form'.
'supported by' means one lot of letters go next to another (letters go below - in down clue).
'lecturer' becomes 'l'.
'form'+'a'+'l'='FORMAL'
(Other definitions for formal that I've seen before include "Ceremonial, solemn" , "stuffy" , "Conforming to accepted rules or customs" , "conventionally correct" , "Essential" .)