The twin's car is at the back of the garage (6)
I believe the answer is:
gemini
'the twin's' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'car is at the back of the garage' is the wordplay.
'car' becomes 'mini' (Mini Cooper).
'is at' means one lot of letters go next to another (I've seen this in other clues).
'the back of the garage' becomes 'ge' (I can't explain this - if you can you should give a lot more credence to this answer).
'mini' after 'ge' is 'GEMINI'.
Can you help me to learn more?
(Other definitions for gemini that I've seen before include "space programme" , "set of twinklers" , "Castor and Pollux constellation" , "House" , "lot of stars" .)