### Clue: Lengthen publicity by circular letter on short London opening (10)
### Answer: PROLONGATE 
 ### Explanation: Definition: Lengthen
 PR(abbrev. for "public relations";publicity) **plus**(by) O(the circular letter of the alphabet) **plus**(on) LON(short for London, in the IATA code for all its airports in general) + GATE(an opening, in a fence, say).