Prize-money we initially kept in safe (6)
I believe the answer is:
pewter
'prize-money' is the definition.
(I've seen this before)
'we initially kept in safe' is the wordplay.
'initially' says to take the initial letters.
'kept in' indicates putting letters inside.
'safe' becomes 'peter' (I've seen this before).
The first letter of 'we' is 'w'.
'w' going inside 'peter' is 'PEWTER'.
(Other definitions for pewter that I've seen before include "Tin/copper (utensils)" , "Plate" , "Grey alloy of tin and lead, makes mugs etc." , "Alloy of tin and lead, often in beer mug" , "Beer-tankard alloy" .)