Speed, beginning to pass point of no return (4)
I believe the answer is:
pace
'speed' is the definition.
(pace is a kind of speed)
'beginning to pass point of no return' is the wordplay.
'beginning to' says to take the initial letters.
'point of no return' becomes 'ace' (tennis serve which is not returned).
The initial letter of 'pass' is 'p'.
'p'+'ace'='PACE'
(Other definitions for pace that I've seen before include "a certain distance" , "Part of step" , "politely expressing disagreement" , "Tempo, rate of moving" , "With the leave of; speed" .)