Hoard, what Bill would keep undisclosed essentially in retirement (5)
I believe the answer is:
cache
'hoard' is the definition.
(both can mean a stock of things)
'what bill would keep undisclosed essentially in retirement' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'bill' becomes 'ac' (short for account).
'would keep' is an insertion indicator.
'essentially' means to look at the middle letters.
'in retirement' is a reversal indicator (retire can mean to go back or backwards).
The centre of 'undisclosed' is 'c'.
'eh'+'ac'='ehac'
'ehac' going around 'c' is 'ehcac'.
'ehcac' written backwards gives 'CACHE'.
(Other definitions for cache that I've seen before include "Concealed store" , "(Place in) hidden store" , "Treasury" , "Secret source" , "Secret hiding place" .)