Farm is iron-clad business, according to a model (9)
I believe the answer is:
franchise
'business according to a model' is the definition.
I can't tell whether this definition defines the answer.
'farm is iron-clad' is the wordplay.
'farm' becomes 'ranch' (ranch is a kind of farm).
'iron' becomes 'fe' (chemical symbol for iron).
'clad' indicates putting letters inside (clad can mean covered by).
'ranch'+'is'='ranchis'
'ranchis' put inside 'fe' is 'FRANCHISE'.
Can you help me to learn more?
(Other definitions for franchise that I've seen before include "The right to vote in elections" , "rights of citizenship" , "Authorisation to sell a company's goods or services in a specified area" , "suffrage" , "Dealership" .)