I take round a thank-you letter (4)

I believe the answer is:

iota

'letter' is the definition.
(iota is a kind of letter)

'i take round a thank-you' is the wordplay.
'take' is a charade indicator (letters next to each other).
'round' becomes 'o' (round can mean a round object).
'a thank you' becomes 'ta'.
'i'+'o'+'ta'='IOTA'

(Other definitions for iota that I've seen before include "Scintilla" , "Grain" , "(Not one) little bit" , "Extremely small amount" , "European letter" .)

I've seen this clue in The Telegraph and The Irish News.

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