Old boy following a thousand in crowd (3)
I believe the answer is:
mob
'crowd' is the definition.
(mob can mean a crowd of people)
'old boy following a thousand in' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'boy' becomes 'b' (abbreviation used in schools).
'following' is a charade indicator (letters next to each other).
'a thousand' becomes 'm' (Roman numeral for a thousand).
'in' means one lot of letters goes inside another.
'b' after 'm' is 'mb'.
'o' going inside 'mb' is 'MOB'.
(Other definitions for mob that I've seen before include "Horde" , "An undisciplined gang of people" , "Unruly crowd" , "Ugly crowd" , "Crowd round" .)