Relief when one gets into pole position (10)
I believe the answer is:
Here is my best explanation:
'relief' is the definition.
'when one gets into pole position' is the subsidiary indication.
'when' becomes 'as'.
'one' becomes 'i' (roman numeral).
'gets into' indicates putting letters inside.
'pole' becomes 's'.
'position' becomes 'stance' (stance is a kind of position**).
'i' placed inside 'sstance' is 'sistance'.
(Other definitions for assistance that I've seen before include "Aid", "Help", "help" and "Support".)