Hint about line in what you are reading (4)
I believe the answer is:
clue
'what you are reading' is the definition.
I can't tell whether this defines the answer.
'hint about line' is the wordplay.
'hint' becomes 'cue' (I've seen this before).
'about' means one lot of letters goes inside another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cue' placed around 'l' is 'CLUE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for clue that I've seen before include "Hint to get what this is?" , "Crossword aid" , "What's this" , "Hint to help the puzzler" , "contribution to crossword" .)