Unusual wheels beneath extremely durable object (5)
I believe the answer is:
demur
'object' is the definition.
(to demur is to object to something)
'unusual wheels beneath extremely durable' is the wordplay.
'unusual' becomes 'rum' (I've seen this in other clues).
'wheels' shows that the letters should be reversed in order (to wheel can mean to turn around).
'beneath' is a charade indicator (letters next to each other) (in a down clue, some letters go beneath others).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'durable' with its middle removed is 'de'.
'rum' backwards is 'mur'.
'mur' put after 'de' is 'DEMUR'.
(Other definitions for demur that I've seen before include "Protest or disagree mildly" , "Cavil, dispute" , "Raise scruples" , "Raise mild objection" , "Protest mildly, object" .)