Disdain shown by Yankee neglecting to depart from Heathrow? (5)
I believe the answer is:
flout
'disdain' is the definition.
(thesaurus)
'yankee neglecting to depart from heathrow?' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'neglecting' is a deletion indicator.
'to depart from heathrow?' becomes 'fly out'.
'flyout' with 'y' taken away is 'FLOUT'.
'shown by' acts as a link.
(Other definitions for flout that I've seen before include "Disobey openly and scornfully" , "Show blatant disregard for" , "Show contempt for (the law)" , "Treat with contempt" , "Show contempt for, defy" .)