Temperature level unknown in old army vehicle going west (7)
I believe the answer is:
subzero
'temperature level' is the definition.
(subzero temperatures)
'unknown in old army vehicle going west' is the wordplay.
'unknown' becomes 'z' (algebra).
'in' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'army vehicle' becomes 'rebus' (I can't explain this - if you can you should believe this answer much more).
'going west' says the letters should be written backwards.
'o'+'rebus'='orebus'
'z' put into 'orebus' is 'orezbus'.
'orezbus' in reverse letter order is 'SUBZERO'.
Can you help me to learn more?
(Other definitions for subzero that I've seen before include "very cold" , "Colder than freezing" , "Negative" , "Extremely nippy" , "Below freezing point" .)