Family of fifty in prison (4)
I believe the answer is:
clan
'family' is the definition.
(I know that clan is a type of family)
'fifty in prison' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'in' indicates putting letters inside.
'prison' becomes 'can' (the can is American slang for prison).
'l' inserted into 'can' is 'CLAN'.
'of' acts as a link.
(Other definitions for clan that I've seen before include "Group with common ancestor, like the Campbells" , "Kinship group" , "Scottish family grouping" , "Tribal family" , "Tribe or collection of families under a cheiftain" .)