Vast spans (4)
I believe the answer is:
eons
'vast spans' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eons that I've seen before include "Long periods of time - noes have it" , "Very long periods (US)" , "Divisions of archaeological time" , "Nose (anag)" , "Years and years" .)