Nut in boat, one heading for Eastbourne (6)
I believe the answer is:
junkie
'nut' is the definition.
(I know that nut can be written as junkie)
'boat one heading for eastbourne' is the wordplay.
'boat' becomes 'junk' (type of Asian ship).
'one' becomes 'i' (Roman numeral).
'heading for' suggests taking the first letters.
The first letter of 'eastbourne' is 'e'.
'junk'+'i'+'e'='JUNKIE'
'in' is the link.
(Other definitions for junkie that I've seen before include "one shooting up" , "Jargon for a drug addict" , "In slang, a drug addict" , "User" .)