In truth, one's totally trustworthy (6)
I believe the answer is:
honest
'trustworthy' is the definition.
(associated in meaning)
'in truth one's totally' is the wordplay.
'in' indicates the answer is hidden within the clue.
'HONEST' is hidden within 'truth ones totally'.
(Other definitions for honest that I've seen before include "Honorable, true" , "really!" , "Law-abiding" , "Impartial intermediary" , "Honourable, true" .)