Fifty youngsters out to form social groups (5)
I believe the answer is:
clubs
'social groups' is the definition.
(club is a kind of social group)
'fifty youngsters out' is the wordplay.
'fifty' becomes 'l' (roman numeral).
'youngsters' becomes 'cubs' (I am not sure about this - if you are sure you should believe this answer much more).
'out' is an insertion indicator.
'l' placed inside 'cubs' is 'CLUBS'.
'to form' acts as a link.
Can you help me to learn more?
(Other definitions for clubs that I've seen before include "Associations of people with similar interests" , "woods, for example" , "Heavy sticks; association" , "Orders" , "For game, you may need five" .)