A single cruise-ship? What a joke! (3-5)
I believe the answer is:
one-liner
'a joke' is the definition.
(I know that one-liner is a type of joke)
'a single cruise-ship?' is the wordplay.
'a single' becomes 'one' (a single thing - only one).
'cruise ship?' becomes 'liner' (I've seen this before).
'one'+'liner'='ONE-LINER'
'what' acts as a link.
(Other definitions for one-liner that I've seen before include "Flat, smooth" , "Quick quip?" , "Terse joke" , "Witty remark consisting of a short sentence" , "Pithy joke" .)