It shows a lack of character, in a word (10)
I believe the answer is:
apostrophe
'word' is the definition.
(thesaurus)
'it shows a lack of character' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'a' is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
'character' could be 'he' (he is a kind of character) and 'he' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "address?" , "Often misplaced punctuation mark" , "At which point discarded letters" , "Mark in exam's" , "Letter omission indicator" .)