Died on boat? That's not fair (4)
I believe the answer is:
dark
'not fair' is the definition.
(as in dark hair, as opposed to fair hair)
'died on boat?' is the wordplay.
'died' becomes 'd'.
'on' means one lot of letters go next to another.
'boat?' becomes 'ark' (ark is a kind of boat).
'd'+'ark'='DARK'
'that's' acts as a link.
(Other definitions for dark that I've seen before include "Devoid of brightness" , "Saturnine" , "Obscure - sinister" , "Gloomy" , "Secret, shadowed" .)