It's often inserted in error, perhaps too inanely (10)
I believe the answer is:
apostrophe
'it's often inserted' is the definition.
I can't judge whether this definition defines the answer.
'error perhaps too inanely' is the wordplay.
'perhaps too' anagrams to 'APOSTROPHE'.
However, I am not clear how this is indicated.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for apostrophe that I've seen before include "Often misplaced punctuation mark" , "in Charley's Aunt ?" , "detail of punctuation" , "Where character may be absent" , "Mark showing omission of letters" .)