Being old man, large ruffian should contain reduced crowd (10)
I believe the answer is:
fatherhood
'being old man' is the definition.
(being a father - your old man is your father)
'large ruffian should contain reduced crowd' is the wordplay.
'large' becomes 'fat' (I've seen this in other clues).
'ruffian' becomes 'hood' (I've seen this in another clue).
'should contain' indicates putting letters inside.
'reduced crowd' becomes 'her' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fat'+'hood'='fathood'
'fathood' placed around 'her' is 'FATHERHOOD'.
Can you help me to learn more?
(Other definitions for fatherhood that I've seen before include "responsibility over issue" , "old man" , "Paternity" .)