Crowd of a thousand meeting old boy (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob is a kind of crowd)
'a thousand meeting old boy' is the wordplay.
'a thousand' becomes 'm' ('M' can be a synonym of 'thousand'**).
'meeting' is a charade indicator (letters next to each other).
'old boy' becomes 'ob'.
'm'+'ob'='MOB'
'of' is the link.
(Other definitions for mob that I've seen before include "press round" , "Rabble" , "Mafiosi" , "Throng" , "Unruly crowd" .)