Tribe of 50 in prison (4)
I believe the answer is:
clan
'tribe' is the definition.
(I know that tribe can be written as clan)
'50 in prison' is the wordplay.
'50' becomes 'l' (Roman numeral).
'in' is an insertion indicator.
'prison' becomes 'can' (the can is American slang for prison).
'l' going within 'can' is 'CLAN'.
'of' is the link.
(Other definitions for clan that I've seen before include "Kin group" , "Kinship group" , "One from Benghazi" , "(Scottish) family group" , "Group of related families" .)