Line of speed (6)
I believe the answer is:
career
I believe this is a double definition.
'line' is the first definition.
(career is a kind of line)
'speed' is the second definition.
(I've seen this before)
'of' acts as a link.
(Other definitions for career that I've seen before include "Dash - profession" , "Run - profession" , "Not intellectual" , "Employment history" , "pursuit" .)