A little sign left in vault (10)
I believe the answer is:
sprinkling
'a little' is the definition.
(a sprinkling is a little of something)
'sign left in vault' is the wordplay.
'sign' becomes 'ink' (ink can mean to sign a contract).
'left' becomes 'L' (common abbreviation).
'in' is an insertion indicator.
'vault' becomes 'spring' (both can mean to jump).
'ink'+'l'='inkl'
'inkl' going inside 'spring' is 'SPRINKLING'.
(Other definitions for sprinkling that I've seen before include "Not much water" , "trickle" , "Drizzling" .)