Filthy square (5)

I believe the answer is:

gross

I believe this is a double definition.

'filthy' is the first definition.
(thesaurus)

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

(Other definitions for gross that I've seen before include "Coarse and crude" , "square?" , "domestic product may be?" , "Repulsively large" , "figure well over a ton" .)

I've seen this clue in The Guardian.

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