In Formula One, he was in favour of finishing fast (5)
I believe the answer is:
prost
'in formula one he was' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'favour of finishing fast' is the wordplay.
'favour of' becomes 'pros' (I can't justify this - if you can you should give a lot more credence to this answer).
'finishing' says to take the final letters (the letter which finishes the word).
The last letter of 'fast' is 't'.
'pros'+'t'='PROST'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for prost that I've seen before include "Toast" , "ex-Formula 1 champion" , "Former French F1 driver" .)