Waste unknown amount in Irish county (6)

I believe the answer is:

offaly

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'irish county' is the definition.
(I've seen this before)

'waste unknown' is the wordplay.
'waste' becomes 'offal' (I've seen this before).
'unknown' becomes 'y' (algebra).
'offal'+'y'='OFFALY'

'amount in' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for offaly that I've seen before include "neighbour of Kildare" , "Irish county" , "A midlands county" .)

I've seen this clue in The Telegraph.

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