It's heard in a haunt possibly at heart of Guadalajara (7)
I believe the answer is:
nahuatl
'it's heard' is the definition.
(indigenous Mexican language)
'a haunt possibly at heart of guadalajara' is the wordplay.
'possibly' indicates anagramming the letters.
'at heart of' means to look at the middle letters (I've seen 'at heart' mean this).
The middle of 'guadalajara' is 'l'.
'a'+'haunt'='ahaunt'
'ahaunt' with letters rearranged gives 'nahuat'.
'nahuat'+'l'='NAHUATL'
'in' is the link.
(Other definitions for nahuatl that I've seen before include "Mexican native language" , "Language of southern Mexico and Central America" , "Aztec" .)