Language of Indians, thus endlessly complicated (10)
I believe the answer is:
hindustani
'language' is the definition.
(I've seen this before)
'indians thus endlessly complicated' is the wordplay.
'endlessly' means to remove the last letter.
'complicated' is an anagram indicator.
'indians'+'thus'='indiansthus'
'indiansthus' with its last letter removed is 'indiansthu'.
'indiansthu' with letters rearranged gives 'HINDUSTANI'.
'of' is the link.
(Other definitions for hindustani that I've seen before include "Indians" , "Language" , "tongue" , "Converting device" , "Dialect of Hindi" .)