Get the lead out in a Roma mostly on Square (5)

I believe the answer is:

smelt

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'square' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I can't explain the rest of the clue.

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(Other definitions for smelt that I've seen before include "Arab leader" , "18 [EXTRACT] metal" , "Food fish often fried" , "Savoured" , "Extract metal from ore, fish" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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