Boat that's a joke following one (5)
I believe the answer is:
liner
'boat' is the definition.
(I've seen this before)
'a joke following one' is the wordplay.
I cannot really understand how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is present in the answer.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for liner that I've seen before include "Removable inner skin or large passenger ship" , "cover for interior" , "QE2, perhaps" , "Big ship" , "Pleasure cruiser" .)