Crossword Genius

Which letter is used twice in this academic submission? (6)

Ross

I believe the answer is:

thesis

'twice in this academic submission?' is the definition.
'thesis' can be an answer for 'academic submission?' (a thesis is a piece of academic work). I am unsure of the rest of the definition.

'which letter is' is the wordplay.
'which' becomes 'the' (I can't explain this - if you can you should believe this answer much more).
'letter' becomes 's'.
'the'+'s'+'is'='THESIS'

'used' acts as a link.

(Other definitions for thesis that I've seen before include "writing following research" , "Long research essay or treatise written for university degree" , "Proposition" , "Essay putting forward a particular view" , "Treatise advancing a new point of view resulting from research" .)

I've seen this clue in The Times.
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