Speed of sportsmen running on pitch, initially (10)

'speed' is the definition.
(promptness is a kind of speed)

'sportsmen running on pitch initially' is the wordplay.
'running' is an anagram indicator.
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'initially' suggests taking the first letters.
The initial letter of 'pitch' is 'p'.
'sportsmen' anagrammed gives 'promtness'.
'promtness' enclosing 'p' is 'PROMPTNESS'.

'of' acts as a link.