No empty space in oval (5)
I believe the answer is:
ovoid
'oval' is the definition.
(synonyms)
'no empty space' is the wordplay.
'no' becomes 'o' (resembles 0 - 'no' can mean 'zero').
'empty space' becomes 'void' (I've seen this before).
'o'+'void'='OVOID'
'in' acts as a link.
(Other definitions for ovoid that I've seen before include "More or less egg-shaped" , "like eggs!" , "rounded shape" , "Like Humpty-Dumpty" , "shaped like rugger ball?" .)