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 before).
'wheels' shows that the letters should be reversed in order (to wheel can mean to turn around).
'beneath' means one lot of letters go next to another (in a down clue).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'durable' with its centre taken out is 'de'.
'rum' back-to-front is 'mur'.
'mur' 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" .)