Some foul, at intervals, language (5)
I believe the answer is:
latin
'language' is the definition.
(Latin is an example)
'some foul at intervals' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'LATIN' can be found hidden inside 'foul at intervals'.
(Other definitions for latin that I've seen before include "demanding subject" , "and 15 Down: Student area of Paris" , "A Romance language" , "Cicero's tongue" , "language class" .)