Sign that reveals a lack of character (10)
I believe the answer is:
apostrophe
This might be a double definition.
'sign that reveals a lack' is the first definition.
'apostrophe' can be an answer for 'sign' (apostrophe is a kind of sign). I'm not sure about the rest of the definition.
'character' is the second definition.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "Mark showing omission of letters" , "address?" , "Where character may be absent" , "It may be superfluous on tradesman's sign" , "rhetorical digression" .)