Tenacious, determined to bring relative in (9)
I believe the answer is:
insistent
'tenacious' is the definition.
(thesaurus)
'determined to bring relative in' is the wordplay.
'determined' becomes 'intent' (I've seen this before).
'to bring relative' becomes 'sis' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'intent' enclosing 'sis' is 'INSISTENT'.
Can you help me to learn more?
(Other definitions for insistent that I've seen before include "Not giving up" , "Demanding attention" , "Determined" , "Compelling attention" , "Refusing to be thwarted" .)