Wally distastefully pens upsetting poem (5)
I believe the answer is:
idyll
'poem' is the definition.
(I've seen this before)
'wally distastefully pens upsetting' is the wordplay.
'pens' indicates the answer is hidden within the clue (pen can mean to contain or cage).
'upsetting' says the letters should be written in reverse.
'llydi' is hidden within 'wally distastefully'.
'llydi' back-to-front is 'IDYLL'.
(Other definitions for idyll that I've seen before include "Pastoral story" , "country work!" , "Happy period" , "Poem or scene of pastoral simplicity" , "Poem describing pastoral scenes" .)