Stern and bow includes heart of ship (6)
I believe the answer is:
behind
'stern' is the definition.
('behind' can be a synonym of 'stern')
'bow includes heart of ship' is the wordplay.
'bow' becomes 'bend' (I've seen this in other clues).
'includes' means one lot of letters goes inside another.
'heart of' means to look at the middle letters.
The middle of 'ship' is 'hi'.
'bend' placed around 'hi' is 'BEHIND'.
'and' is the link.
(Other definitions for behind that I've seen before include "Responsible for" , "Taking place in secret" , "Hidden by; in arrears" , "Running late" , "Hidden by; late" .)