The deepest as part by the majority (6)
I believe the answer is:
'the deepest' is the definition.
(I've seen this in another clue)
'part by the majority' is the wordplay.
'part' becomes 'in' (synonyms).
'by' means one lot of letters go next to another.
'the majority' becomes 'most' (most of a group).
'as' acts as a link.
(Other definitions for inmost that I've seen before include "Being deepest within the self" , "Deepest within oneself" , "Situated or occurring farthest within" , "Furthest inwards, closest to centre" , "Furthest inward" .)