Fine short facial hair in an instant (5)
I believe the answer is:
flash
'an instant' is the definition.
(I know that instant can be written as flash)
'fine short facial hair' is the wordplay.
'fine short' becomes 'f' (abbreviation in pencil classifications; 'short' suggests an abbreviation).
'facial hair' becomes 'lash' (I've seen this in another clue).
'f'+'lash'='FLASH'
'in' is the link.
(Other definitions for flash that I've seen before include "short space of time" , "Briefly shine, short flood" , "Streak" , "Instant" , "Sudden gleam of light" .)