That which is more likely to reduce speed (5)
I believe the answer is:
haste
'that which is more likely' is the definition.
I can't judge whether this definition defines the answer.
'reduce speed' is the wordplay.
'reduce' means to remove the last letter (I've seen 'reduced' mean this).
'speed' becomes 'hasten' (synonyms).
'hasten' with its last letter removed is 'HASTE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for haste that I've seen before include "Speed, hurry" , "Rash speed" , "Urgency of action" , "Leader abandons modest" , "Speediness or rush" .)