Slender object? Not entirely (4)
I believe the answer is:
thin
'slender' is the definition.
(synonyms)
'object? not entirely' is the wordplay.
'object?' becomes 'thing' (an object is a thing or item).
'not entirely' means to remove the last letter (not all of the word).
'thing' with its last letter taken away is 'THIN'.
(Other definitions for thin that I've seen before include "Implausible" , "as 7 [LOAF] may be sliced" , "Unconvincing" , "Carrying little fat" , "Flimsy" .)