Car firm opens plant — it has ups and downs! (6,7)
I believe the answer is:
roller coaster
'it has ups and downs' is the definition.
The definition suggests a singular noun which matches the answer.
'car firm opens plant' is the wordplay.
I cannot really understand how this works, but
'car' could be 'roller' (I've seen this before) and 'roller' is found in the answer.
'firm' could be 'co' (a firm is a company) and 'co' is found in the answer.
'plant' could be 'aster' (aster is a kind of plant) and 'aster' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for roller coaster that I've seen before include "Ups and downs on this" , "Big dipper" , "Series of emotional swings" , "Fairground ride" , ""Amusement" ride" .)