Idly work on first long poem (5)
I believe the answer is:
idyll
'poem' is the definition.
(thesaurus)
'idly work on first long' is the wordplay.
'work' indicates anagramming the letters.
'on' is an insertion indicator (as in clothing \'on\' a person).
'first' indicates taking the first letters.
The initial letter of 'long' is 'l'.
'idly' anagrammed gives 'idyl'.
'idyl' going 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" .)