Racing at Epsom — number of spectators squeezed in (4)
I believe the answer is:
gate
'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'racing at epsom number of spectators squeezed' is the wordplay.
'GATE' is hidden in amongst the letters.
But, I am uncertain how the hidden word is indicated.
(Other definitions for gate that I've seen before include "Movable opening" , "Barrier - door" , "Frame for closing an entrance" , "Garden door?" , "Spectators - entrance" .)