Poet composing sonnet about New York (8)
'poet' is the definition.
(Alfred, Lord Tennyson)
'composing sonnet about new york' is the wordplay.
'composing' is an anagram indicator.
'about' means one lot of letters goes inside another.
'new york' becomes 'ny'.
'sonnet' is an anagram of 'tenson'.
'tenson' enclosing 'ny' is 'TENNYSON'.
(Other definitions for tennyson that I've seen before include "English 10 across", "Great Victorian", "C19 Poet Laureate", "literary lord", "linesman".)