Idly work on first long poem (5)
I believe the answer is:
idyll
'poem' is the definition.
(type of poem)
'idly work on first long' is the wordplay.
'work' indicates anagramming the letters.
'on' indicates putting letters inside (as in clothing 'on' a person).
'first long' becomes 'l' (1st letter of 'long').
'idly' anagrammed gives 'idyl'.
'idyl' placed around 'l' is 'IDYLL'.
(Other definitions for idyll that I've seen before include "Picturesque rural scene" , "Happy period" , "Short pastoral poem" , "Poem describing pastoral scenes" , "piece of poetry" .)