For each part of poem is difficult (8)
I believe the answer is:
perverse
'difficult' is the definition.
(I've seen this before)
'for each part of poem' is the wordplay.
'for each' becomes 'per' (I've seen this before).
'part of poem' becomes 'verse' ('verse' is part of 'poem').
'per'+'verse'='PERVERSE'
'is' acts as a link.
(Other definitions for perverse that I've seen before include "Deliberately difficult" , "Deliberately unreasonable" , "Cross-grained" , "Irregular" , "'Unreasonable, awkward (8)'" .)