Horizontal surface in which line meets side (5)
I believe the answer is:
ledge
'horizontal surface in which' is the definition.
'ledge' can be an answer for 'horizontal' (thesaurus). I'm not sure about the rest of the definition.
'line meets side' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'meets' means one lot of letters go next to another.
'side' becomes 'edge' (edge is a kind of side).
'l'+'edge'='LEDGE'
Can you help me to learn more?
(Other definitions for ledge that I've seen before include "Surface under e.g. window" , "Narrow projection or shelf" , "Rock shelf" , "rocky situation?" , "Shelf-like projection" .)