得意 here is the old sense "a regular patron", as in お得意さん and お得意様, not "good at".
Common collocations
- 長年の得意客: a long-standing customer
- 得意客を失う: to lose a valued customer
- お得意様: valued customer (polite, used by the business itself)
Similar words
- 常連: a regular, stressing frequent visits (bars, restaurants)
- 顧客: customer, client (formal, business)