One wizard line in poetic language? (7)
I believe the answer is:
imagery
'poetic language?' is the definition.
'one wizard line' is the wordplay.
'one' becomes 'i' (Roman numeral).
'wizard' becomes 'mage'.
'line' becomes 'ry' (abbreviation for railway).
'i'+'mage'+'ry'='IMAGERY'
'in' acts as a link.
(Other definitions for imagery that I've seen before include "Vivid written or oral descriptions" , "I'm gayer (anag)" , "Figures of speech" , "Mental visions" , "paintings" .)