Home investor in British Gas covers debts advancing gradually (9)
I believe the answer is:
insidious
'advancing gradually' is the definition.
I can't tell whether this defines the answer.
'home investor in british gas covers debts' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'investor in british gas covers' becomes 'sid' (I can't justify this - if you can you should believe this answer much more).
'debts' becomes 'IOUs' (promises to pay debts).
'in'+'sid'+'ious'='INSIDIOUS'
Can you help me to learn more?
(Other definitions for insidious that I've seen before include "Slowly having a harmful effect" , "Treacherous; subtle" , "Beguiling but harmful" , "Gradually harmful" , "Treacherously sly" .)