Round hole is egg-shaped (5)
I believe the answer is:
ovoid
'is egg-shaped' is the definition.
(synonyms)
'round hole' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'hole' becomes 'void'.
'o'+'void'='OVOID'
(Other definitions for ovoid that I've seen before include "shaped like rugger ball?" , "rounded shape" , "Elliptical" , "shaped like distorted circle" , "Rounded like an egg" .)