Sailors in flat-bottomed boat showing heart (6)
I believe the answer is:
kernel
'heart' is the definition.
(synonyms)
'sailors in flat-bottomed boat' is the wordplay.
'sailors' becomes 'RN' (abbreviation for Royal Navy).
'in' means one lot of letters goes inside another.
'flat-bottomed boat' becomes 'keel' (type of boat).
'rn' inserted inside 'keel' is 'KERNEL'.
'showing' is the link.
(Other definitions for kernel that I've seen before include "Heart and soul" , "Central or most important part" , "Edible seed of nut" , "essentials" , "Contents of shell" .)