Those heading for airboat relay some impolite language (5)
I believe the answer is:
farsi
'language' is the definition.
(Farsi is an example)
'those heading for airboat relay some impolite' is the wordplay.
'those heading' indicates taking the first letters (the heads of the words).
The initial letters of 'for airboat relay some impolite' is 'FARSI'.
(Other definitions for farsi that I've seen before include "Language of Iran" , "Persian language" , "Modern Persian lang." , "Iranian language, also widely spoken in Afghanistan" , "foreign language" .)