Ring useless -- it's egg-shaped (5)
I believe the answer is:
ovoid
'egg-shaped' is the definition.
(synonyms)
'ring useless' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'useless' becomes 'void'.
'o'+'void'='OVOID'
'it's' acts as a link.
(Other definitions for ovoid that I've seen before include "More or less egg-shaped" , "rounded shape" , "Shaped like an egg" , "shaped like distorted circle" , "Rounded like an egg" .)