Filled out, invoice outstanding (8)

I believe the answer is:

billowed

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'filled out' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Maybe they are linked in a way I don't understand?

'invoice outstanding' is the wordplay.
'invoice' becomes 'bill' (synonyms).
'outstanding' becomes 'owed' (as in an outstanding debt).
'bill'+'owed'='BILLOWED'

Can you help me to learn more?

(Other definitions for billowed that I've seen before include "Flowed with an undulating motion; filled with air" , "Swollen account was due payment" , "was swelling" .)

I've seen this clue in The Telegraph.

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