Sign for the letter that's gone missing (10)
I believe the answer is:
apostrophe
'sign' is the definition.
(apostrophe is a kind of sign)
'the letter that's gone missing' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'letter' could be 'he' (he is a kind of letter) and 'he' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "detail of punctuation" , "Letter omission indicator" , "rhetorical digression" , "It may be superfluous on tradesman's sign" , "Part of man's" .)