Writer one charged to invest bit of spare cash for retirement (7)
I believe the answer is:
pension
'cash for retirement' is the definition.
(money supplied in old age)
'writer one charged to invest bit of spare' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'one charged' becomes 'ion' (type of charged particle).
'to invest' indicates putting letters inside (back).
'bit of' indicates taking the first letters.
The initial letter of 'spare' is 's'.
'pen'+'ion'='penion'
'penion' placed around 's' is 'PENSION'.
(Other definitions for pension that I've seen before include "residence abroad" , "Payment to retired persons a continental boarding house" , "Regular payment for older people" , "Bed and breakfast or small hotel in France" , "Payment to traitor" .)