Poem I would finish in twenty lines (5)
I believe the answer is:
idyll
'poem' is the definition.
(type of poem)
'i would finish in twenty lines' is the wordplay.
'i would' becomes 'I'd'.
'finish in' indicates one should take the final letters.
'lines' becomes 'll' (two abbreviations for line).
The last letter of 'twenty' is 'y'.
'id'+'y'+'ll'='IDYLL'
(Other definitions for idyll that I've seen before include "Episode of romantic charm" , "Blissful, rustic, scene" , "Episode of happy innocence" , "Bucolic state" , "Short pastoral poem" .)