For every section of the poem is unreasonable (8)
I believe the answer is:
perverse
'unreasonable' is the definition.
(I've seen this before)
'for every section' is the wordplay.
'for every' becomes 'per' ('per' means 'for each' or 'for every').
'section' becomes 'verse' (I have seen 'Section of poem ' mean 'verse' so perhaps 'section' could also mean 'verse').
'per'+'verse'='PERVERSE'
'of the poem is' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for perverse that I've seen before include "Obstinately wrong-headed" , "Deliberately contrary" , "Deliberately unreasonable" , "'Contrary to what's considered right, proper or good (8)'" , "Cross-grained" .)