Object ultimately going spare (4)
I believe the answer is:
thin
'spare' is the definition.
(similar in meaning)
'object ultimately going' is the wordplay.
'object' becomes 'thing' (an object is a thing or item).
'ultimately going' means to remove the last letter (the last or ultimate letter is removed).
'thing' with its last letter taken away is 'THIN'.
(Other definitions for thin that I've seen before include "Insubstantial" , "Spare" , "Slender, slight" , "Scrawny" , "Having little width" .)