Sailors entering part of ship? That’s the important point (6)
I believe the answer is:
kernel
'the important point' is the definition.
(kernel can mean the central point)
'sailors entering part of ship?' is the wordplay.
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'entering' indicates putting letters inside.
'part of ship?' becomes 'keel' (I've seen this before).
'rn' placed within 'keel' is 'KERNEL'.
'that's' acts as a link.
(Other definitions for kernel that I've seen before include "Edible seed of nut" , "Essential part" , "Inner part of a seed, the heart of the matter" , "Stone" , "soft heart" .)