Large family; many in jail (4)
I believe the answer is:
clan
'large family' is the definition.
(I've seen this before)
'many in jail' is the wordplay.
'many' becomes 'l'.
'in' is an insertion indicator.
'jail' becomes 'can' (the can is American slang for prison).
'l' placed inside 'can' is 'CLAN'.
(Other definitions for clan that I've seen before include "Large family forming close group, big in Scotland" , "(Scottish) family group" , "Group with common ancestor, like the Campbells" , "Scottish tribe" , "Group of related families" .)