Always in one's thoughts (2,3,5)
'always in' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for telepathic that I've seen before include "Communicating by mind alone" , "Extraordinarily communicative" , "Able to pick up ideas" , "communicating senselessly?" , "Communicating by thought" .)
(Another definition for on the brain that I've seen is " constantly in mind".)