Sarah and I bought a chestnut (7)

I believe the answer is:

saligot

'a chestnut' is the definition.
(Trapa natans)

'sarah and i bought' is the wordplay.
'sarah' becomes 'sal' (Sally is a common pet form of Sarah).
'and' says to put letters next to each other.
'bought' becomes 'got'.
'sal'+'i'+'got'='SALIGOT'

I've seen this clue in The Telegraph.

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