Just a single ship? That’s a joke (3-5)
I believe the answer is:
one-liner
'that's a joke' is the definition.
(I've seen this before)
'just a single ship?' is the wordplay.
'just a single' becomes 'one' (only one).
'ship?' becomes 'liner' (liner is a kind of ship).
'one'+'liner'='ONE-LINER'
(Other definitions for one-liner that I've seen before include "Witty remark in single short sentence" , "Flat, smooth" , "Short joke - witty remark" , "Single sentence jokes" , "Terse joke" .)