Fine to disappear in robbery, a letter (5)

I believe the answer is:

theta

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'letter' is the definition.
(theta is a kind of letter of the alphabet)

I don't understand the remainder of the clue.

(Other definitions for theta that I've seen before include "classic character?" , "Greek character" , "Letter to the Corinthians" , "Greek letter equivalent to th" , "Letter before iota" .)

I've seen this clue in The Telegraph.

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