A mail line including hotel’s address (10)
I believe the answer is:
apostrophe
'address' is the definition.
(I know that apostrophe is a type of address)
'a mail line including hotel's' is the wordplay.
'mail' becomes 'post' (synonyms).
'line' becomes 'rope' (line can mean a piece of rope).
'including' is an insertion indicator.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'rope' going around 'h' is 'rophe'.
'a'+'post'+'rophe'='APOSTROPHE'
(Other definitions for apostrophe that I've seen before include "rhetorical digression" , "At which point discarded letters" , "Where character may be absent" , "Mark in exam's" , "detail of punctuation" .)