ABC air foul language (6)
I believe the answer is:
arabic
'language' is the definition.
(Arabic is a language)
'abc air foul' is the wordplay.
'foul' is an anagram indicator.
'abc'+'air'='abcair'
'abcair' with letters rearranged gives 'ARABIC'.
(Other definitions for arabic that I've seen before include "Common language of Middle East" , "Language (of numbers)" , "Gum --, ingredient of glue and incense" , "Major language of Middle East" , "foreign language" .)