There's nothing in solid secret store (5)
I believe the answer is:
hoard
'secret store' is the definition.
(I've seen this in another clue)
'there's nothing in solid' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in' is an insertion indicator.
'solid' becomes 'hard' (similar in meaning).
'o' placed into 'hard' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Store of money or valuables" , "Secret store of valuables" , "Secret stash" , "One's savings" , "accumulated wealth" .)