Many a dirty dog's expression of displeasure (4)

I believe the answer is:

drat

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'many a dirty dog's expression of displeasure' is the definition.
I can't tell whether this definition defines the answer.

'many a dirty dog's' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'a dirty dog' becomes 'rat' (I've seen this in another clue).
'd'+'rat'='DRAT'

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(Other definitions for drat that I've seen before include "Expression of annoyance on DART" , "Informal expression of annoyance" , "Mild expletive" , "Exclamation of annoyance" , "Darn it!" .)

I've seen this clue in the Evening Standard.

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