Feature of many a poem? "Hard to fill lines" - the writer (5)
I believe the answer is:
rhyme
'feature of many a poem?' is the definition.
I can't tell whether this definition defines the answer.
'hard to fill lines the writer' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to fill' means one lot of letters goes inside another.
'lines' becomes 'ry' (abbreviation for railway).
'the writer' becomes 'me' (the person who wrote the clues).
'h' placed inside 'ry' is 'rhy'.
'rhy'+'me'='RHYME'
Can you help me to learn more?
(Other definitions for rhyme that I've seen before include "Correspondence in the sounds of two or more words" , "something read" , "Or cost in relation to it?" , "Verse of poetry" , "Piece of poetry" .)