Bovine material in test mostly upheld as source of ventilation (3,4)

'bovine material' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

'test mostly upheld as source of ventilation' is the wordplay.
I cannot quite understand how this works, but
'ventilation' could be 'air' (I've seen this in another clue) and 'air' is present in the answer.
The remaining letters 'duct' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'in' is the link.

Can you help me to learn more?