Deepest within the self (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
inmost
(Other definitions for inmost that I've seen before include "Being deepest within the self" , "Furthest inward" , "Situated or occurring farthest within" , "Nearest centre; deepest" , "Furthest inwards, closest to centre" .)