Clue: Measure of interest in group of islands, or one (5)
Answer: CAPRI
Explanation: Definition: one 
 APR(abbrev. for "annual percentage rate", the amount/measure of interest generated by a sum of money, paid by a borrower to a lender) **contained in**(in) CI(abbrev. for the Channel Islands, a group of islands in the English Channel).
Defn: … island.