Quantities of cargo, eight in top fire-resistant storage (8)
I believe the answer is:
freights
'quantities of cargo' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'eight in top fire-resistant storage' is the wordplay.
'in' indicates putting letters inside.
'top' says to take the initial letters (in a down clue, the first letter is top).
The initial letters of 'fire resistant storage' is 'frs'.
'eight' put into 'frs' is 'FREIGHTS'.
Can you help me to learn more?
(Another definition for freights that I've seen is " Transports".)