Poet having something wonderful with extra line (7)
I believe the answer is:
marvell
'poet' is the definition.
(17th-century poet Andrew Marvell)
'something wonderful with extra line' is the wordplay.
'something wonderful' becomes 'marvel' (a marvel is a wonderful or miraculous thing).
'with extra' is a charade indicator (letters next to each other).
'line' becomes 'l' (used when specifying particular lines from a poem).
'marvel'+'l'='MARVELL'
'having' acts as a link.
(Other definitions for marvell that I've seen before include "Andrew -, seventeenth century English poet" , "Andrew -, English metaphysical poet" , "Poet who wrote To His Coy Mistress" .)