Clue: Immigration of unknown number into 15 port, I gathered — is that feared by 15? (10)
Answer: KRYPTONITE
Explanation: Definition: is that feared by 15?
An envelope ('immigration … into') of Y ('unknown number') in KRPTONITE, an anagram ('gathered') of KENT ('15') plus 'port I'. The '15' in the definition is, for a change, Clark KENT, Superman.