Like an egg, but an egg with nothing inside (5)
I believe the answer is:
ovoid
'like an egg' is the definition.
(oval-shaped or egg-shaped)
'an egg with nothing inside' is the wordplay.
'an egg' becomes 'o' (an 'o' resembles an egg).
'with nothing inside' becomes 'void' (void can mean empty).
'o'+'void'='OVOID'
'but' acts as a link.
(Other definitions for ovoid that I've seen before include "Like Humpty-Dumpty" , "Elliptical" , "shaped like rugger ball?" , "More or less egg-shaped" , "like eggs!" .)