Rapid growth of port airline being developed (13)
I believe the answer is:
proliferation
'rapid growth' is the definition.
'of port airline being developed' is the wordplay.
'being developed' is an anagram indicator.
'of'+'port'+'airline'='ofportairline'
'ofportairline' is an anagram of 'PROLIFERATION'.
(Other definitions for proliferation that I've seen before include "Increase (of nuclear weapons)" , "Increase in numbers" .)