Unknown bird's cry (4)

I believe the answer is:

yowl

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

'unknown bird's' is the wordplay.
'unknown' becomes 'y' (algebra).
'bird' becomes 'owl' (owl is a kind of bird).
'y'+'owl'='YOWL'

(Other definitions for yowl that I've seen before include "Wail" , "Wailing cry (not howl)" .)

I've seen this clue in The Independent.

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