Hint — what you're reading is missing line (3)
I believe the answer is:
cue
'hint' is the definition.
(thesaurus)
'what you're reading is missing line' is the wordplay.
'what you're reading' becomes 'clue'.
'is missing' indicates named letters should be taken away.
'line' becomes 'l'.
'clue' with 'l' removed is 'CUE'.
(Other definitions for cue that I've seen before include "Sign" , "line in speech" , "Rod used in billiards" , "Signal for action" , "On a car[D?] this would give a prompt" .)