A part of poem's hostile (6)
I believe the answer is:
averse
'hostile' is the definition.
(thesaurus)
'a part of poem's' is the wordplay.
'part of poem' becomes 'verse' ('verse' is part of 'poem').
'a'+'verse'='AVERSE'
(Other definitions for averse that I've seen before include "'Opposed, disinclined (6)'" , "Metal rod; card game" , "Disinclined - as ever (anag)" , "Having a strong opposition (to)" , "Antipathetic" .)