Was it spoken in U-boats? (6)

I believe the answer is:

basuto

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'was it spoken' is the definition.
I can't tell whether this defines the answer.

'u-boats?' is the wordplay.
'u-boats' anagrams to 'BASUTO'.
However, I'm not sure how this is indicated.

'in' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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