Poet essentially pens 500 lines
I believe the answer is:
ode
'lines' is the definition.
(I've seen this before)
'poet essentially pens 500' is the wordplay.
'essentially' says to take the centre.
'pens' means one lot of letters goes inside another.
'500' becomes 'D' (Roman numeral for 500).
The middle of 'poet' is 'oe'.
'oe' enclosing 'd' is 'ODE'.
(Other definitions for ode that I've seen before include "A poem celebrating person or event" , "Poetic composition" , "Poem expressing noble feelings" , "Lyric poem" , "(Railway)system" .)