Round, empty space, like a rugby ball (5)
I believe the answer is:
ovoid
'like a rugby ball' is the definition.
(I've seen this in another clue)
'round empty space' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'empty space' becomes 'void' (I've seen this before).
'o'+'void'='OVOID'
(Other definitions for ovoid that I've seen before include "shaped like distorted circle" , "shaped like rugger ball?" , "like eggs!" , "rounded shape" , "Like Humpty-Dumpty" .)