Set down almost half a dozen in storehouse (7)
I believe the answer is:
deposit
'set down' is the definition.
(I've seen this before)
'almost half a dozen in storehouse' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'half a dozen' becomes 'six' ('six' can be a synonym of 'half a dozen').
'in' indicates putting letters inside.
'storehouse' becomes 'depot' ('depot' can be a synonym of 'storehouse').
'six' with its last letter taken away is 'si'.
'si' placed into 'depot' is 'DEPOSIT'.
(Other definitions for deposit that I've seen before include "secure an item" , "Put down - initial payment" , "fur, possibly" , "store" , "Hoard" .)