Crowd alongside a stone (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'crowd alongside a' is the wordplay.
'crowd' becomes 'gate' (gate can refer to the crowd for a sporting event).
'alongside' says to put letters next to each other.
'gate' after 'a' is 'AGATE'.
(Other definitions for agate that I've seen before include "Quartz variety" , "Stone with coloured bands" , "On the way" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "marble" .)