Freighter, transport or cab carrying unknown quantity (6)
I believe the answer is:
boxcar
'freighter' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'transport or cab carrying unknown quantity' is the wordplay.
'transport' is an anagram indicator (letters are transported elsewhere).
'carrying' is an insertion indicator.
'unknown quantity' becomes 'x' (as in maths, or e.g. 'Mr X').
'or'+'cab'='orcab'
'orcab' with letters rearranged gives 'bocar'.
'bocar' placed around 'x' is 'BOXCAR'.
Can you help me to learn more?
(Other definitions for boxcar that I've seen before include "Freight wagon (US)" , "US railway wagon" .)