Opposite, one new poem (7)
I believe the answer is:
inverse
'opposite' is the definition.
(synonyms)
'one new poem' is the wordplay.
'one' becomes 'i' (Roman numeral).
'new' becomes 'n' (abbreviation as in New Testament NT).
'poem' becomes 'verse' (verse is a kind of poem).
'i'+'n'+'verse'='INVERSE'
(Other definitions for inverse that I've seen before include "Opposite (like poetry?)" , "Something the exact opposite of another" , "Opposite in direction" , "Antonym" , "Reversed in order or relation" .)