Old and useless like an egg (5)
I believe the answer is:
ovoid
'like an egg' is the definition.
(oval-shaped or egg-shaped)
'old and useless' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'and' is a charade indicator (letters next to each other).
'useless' becomes 'void'.
'o'+'void'='OVOID'
(Other definitions for ovoid that I've seen before include "More or less egg-shaped" , "shaped like distorted circle" , "like eggs!" , "Eggy" , "shaped like rugger ball?" .)