Start to value a means of saving official document (4)
I believe the answer is:
visa
'official document' is the definition.
(travel document)
'start to value a means of saving' is the wordplay.
'start to' says to take the initial letters.
'a means of saving' becomes 'ISA' (Individual Savings Account).
The initial letter of 'value' is 'v'.
'v'+'isa'='VISA'
(Other definitions for visa that I've seen before include "Stamp affixed to passport as permission to enter country" , "ID" , "travel authority" , "Travel permit stamped in passport etc" , "Authorisation to enter a country" .)