Poet's line included in course (5)
I believe the answer is:
plath
'poet's' is the definition.
(I've seen this before)
'line included in course' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'included in' indicates putting letters inside.
'course' becomes 'path' (path can mean a route or course).
'l' placed inside 'path' is 'PLATH'.
(Other definitions for plath that I've seen before include "See 17" , "Sylvia --, American poet" , "Sylvia -, US poet, married Ted Hughes" .)