Reserved object, last in sale (6)
I believe the answer is:
demure
'reserved' is the definition.
(thesaurus)
'object last in sale' is the wordplay.
'object' becomes 'demur'.
'last in' indicates one should take the final letters.
The last letter of 'sale' is 'e'.
'demur'+'e'='DEMURE'
(Other definitions for demure that I've seen before include "butter not melting in mouth?" , "Shy, prim" , "Coy, shy" , "Pretending to be quiet and serious" , "Affectedly modest or shy" .)