Hot lines? Quite the opposite (7)
I believe the answer is:
inverse
'quite the opposite' is the definition.
(inverse means something's opposite)
'hot lines?' is the wordplay.
'hot' becomes 'in' (both can mean fashionable).
'lines?' becomes 'verse' (lines of poetry).
'in'+'verse'='INVERSE'
(Other definitions for inverse that I've seen before include "The exact opposite of enviers" , "Upside down - opposite in effect" , "Something the exact opposite of another" , "Antonym" , "Opposite (like poetry?)" .)