Sport’s coming back with zero crowds (6)
I believe the answer is:
troops
'crowds' is the definition.
(troop is a kind of crowd)
'sport's coming back with zero' is the wordplay.
'coming back' shows that the letters should be reversed in order.
'with' indicates putting letters inside.
'zero' becomes 'o'.
'sport' back-to-front is 'trops'.
'trops' going around 'o' is 'TROOPS'.
(Other definitions for troops that I've seen before include "Army" , "Cavalry" , "Military forces" , "men" , "Bodies of soldiers" .)