Line in what you're reading to classes (6)
I believe the answer is:
worlds
'to classes' is the definition.
(world is a kind of class)
'line in what you're reading' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' means one lot of letters goes inside another.
'what you're reading' becomes 'words' (you're reading words!).
'l' inserted inside 'words' is 'WORLDS'.
(Other definitions for worlds that I've seen before include "Universes" , "planets" .)