End of lesson in silence for badly behaved group? (4)
I believe the answer is:
gang
'badly behaved group?' is the definition.
(criminal gang)
'end of lesson in silence' is the wordplay.
'end of' says to take the final letters.
'in' indicates putting letters inside.
'silence' becomes 'gag' (gagging is a kind of silencing).
The final letter of 'lesson' is 'n'.
'n' placed inside 'gag' is 'GANG'.
'for' acts as a link.
(Other definitions for gang that I've seen before include "Group; clique" , "Men working" , "go north of the border" , "Working group or mob of hoodlums" , "A lot of labourers" .)