Sent to have a fill with zero capital (10)
I believe the answer is:
sacramento
'capital' is the definition.
(state capital of California)
'sent to have a fill with zero' is the wordplay.
'to have' is an insertion indicator.
'fill' becomes 'cram' (synonyms).
'with' means one lot of letters go next to another.
'zero' becomes 'O' (O resembles 0).
'a'+'cram'='acram'
'sent' going around 'acram' is 'sacrament'.
'sacrament'+'o'='SACRAMENTO'
(Other definitions for sacramento that I've seen before include "port" , "State capital of California" , "US city" .)