Some secret, one turning up in short letter? (4)
I believe the answer is:
note
'short letter?' is the definition.
('note' can be a synonym of 'short letter')
'some secret one turning up' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'turning up' says the letters should be written in reverse.
'eton' can be found hidden inside 'secret one'.
'eton' reversed gives 'NOTE'.
'in' is the link.
(Other definitions for note that I've seen before include "Take this to heed" , "Tone of a short record" , "Eminence" , "Fame - memorandum" , "Observe the musical pitch." .)