In such a car, a rear window is unusual (4)
I believe the answer is:
rare
'unusual' is the definition.
(rare can mean unusual or odd)
'in such a car a rear' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RARE' is hidden within 'such car a rear'.
'window is' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "Especially good" , "Choice - lightly cooked - thin" , "Very infrequent, lightly cooked" , "Not found often" , "Scarcely seen" .)