Incredible tact in entertaining one liner (7)
I believe the answer is:
titanic
'liner' is the definition.
(I've seen this before)
'incredible tact in entertaining one' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for titanic that I've seen before include "made of metal" , "Of enormous size, like the stricken liner" , "Successful movie - disaster ship" , "Famously sunken ship" , "Twenty one [SHIP]" .)