Vast number briefly concealing a cache (5)
I believe the answer is:
hoard
'cache' is the definition.
(synonyms)
'vast number briefly concealing a' is the wordplay.
'vast number' becomes 'horde' (I've seen this in another clue).
'briefly' means to remove the last letter.
'concealing' means one lot of letters goes inside another.
'horde' with its final letter removed is 'hord'.
'hord' placed around 'a' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Hard kick" , "A lot of people" , "Valuable store" , "Squirrel away" , "Secret stash" .)