Core group of mariners trapped by bottom of boat (6)
I believe the answer is:
kernel
'core group' is the definition.
'kernel' can be an answer for 'core' (I've seen this before). I am not certain of the 'group' bit.
'mariners trapped by bottom of boat' is the wordplay.
'mariners' becomes 'RN' (Royal Navy).
'trapped by' means one lot of letters goes inside another.
'bottom of boat' becomes 'keel' (I've seen this before).
'rn' going within 'keel' is 'KERNEL'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for kernel that I've seen before include "Seed in hard shell" , "Contents of shell" , "Edible or central part of nut" , "Nuts and bolts" , "Essential part" .)