Nervous runner times his running (8)
I believe the answer is:
skittish
'nervous' is the definition.
('skittish' can be a synonym of 'nervous')
'runner times his running' is the wordplay.
'runner' becomes 'ski' (ski is a kind of runner**).
'times' becomes 'tt' (two abbreviations for time).
'running' is an anagram indicator.
'his' anagrammed gives 'ish'.
'ski'+'tt'+'ish'='SKITTISH'
(Other definitions for skittish that I've seen before include "Frisky or playful (8)." , "Frivolous" , "Playful, hard to control" , "Frisky or mettlesome" , "Playful in coy manner, frisky" .)