Train’s erratic temperature in carriage (7)
I believe the answer is:
transit
'carriage' is the definition.
(I've seen this in other clues)
'train's erratic temperature in' is the wordplay.
'erratic' is an anagram indicator.
'temperature' becomes 't' (abbreviation).
'in' indicates putting letters inside.
'trains' anagrammed gives 'ransit'.
'ransit' going around 't' is 'TRANSIT'.
(Other definitions for transit that I've seen before include "Crossing - passage" , "Passage of heavenly body" , "The process of going or passing - sic . . . . . . . gloria mundi" , "Movement (of goods)" , "Act of passing across" .)