Mostly useless runner failing to finish in twilight (4)
I believe the answer is:
dusk
'twilight' is the definition.
(I know that dusk is a type of twilight)
'mostly useless runner failing to finish' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'useless' becomes 'dud' (dud can mean faulty or failing).
'runner' becomes 'ski' (something which 'runs', ie moves quickly on snow).
'failing to finish' means to remove the last letter.
'dud' with its last letter taken away is 'du'.
'ski' with its last letter removed is 'sk'.
'du'+'sk'='DUSK'
'in' is the link.
(Other definitions for dusk that I've seen before include "Evening light" , "Period after sunset" , "Nightfall" , "Gloaming" , "Sundown" .)