The poet's always under the car of course (6)
I believe the answer is:
'course' is the definition.
(I've seen this in another clue)
'the poet's always under the car' is the wordplay.
'the poet's always' becomes 'eer' ('e'er' is a poetic term for 'ever' or 'always').
'under' says to put letters next to each other (in a down clue, some letters go below others).
'eer' after 'car' is 'CAREER'.
'of' is the link.
(Other definitions for career that I've seen before include "series of jobs" , "Progress in occupation" , "Lifetime occupation" , "Run out of control - job for life?" , "Progress of working life" .)