Moisten everything, in case (6)
I believe the answer is:
wallet
'case' is the definition.
(wallet is a kind of case)
'moisten everything in' is the wordplay.
'moisten' becomes 'wet' (I've seen this in another clue).
'everything' becomes 'all' (all things).
'in' indicates putting letters inside.
'wet' placed around 'all' is 'WALLET'.
(Other definitions for wallet that I've seen before include "Small flat money case" , "ready to come out of it?" , "Billfold (US)" , "Pocket-size case" , "Folding case for money" .)