Poet's key lines needing study (6)
I believe the answer is:
dryden
'poet's' is the definition.
(John Dryden)
'key lines needing study' is the wordplay.
'key lines' becomes 'dry' (I can't explain this - if you can you should believe this answer much more).
'needing' means one lot of letters go next to another.
'study' becomes 'den' (den can mean an office or study).
'dry'+'den'='DRYDEN'
(Other definitions for dryden that I've seen before include "Restoration author" , "17th century English poet and dramatist" , "John --, English 17th-cent. poet/dramatist" , "English Restoration poet" , "Poet laureate, 1668-1689" .)