Legend that’s part of my theories (4)
I believe the answer is:
myth
'legend' is the definition.
(I've seen this before)
'part of my theories' is the wordplay.
I cannot really see how this works, but
'my' is found in the answer.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for myth that I've seen before include "Common false belief" , "Illusion" , "Fiction" , "Fictitious thing; traditional story" , "lie!" .)