They're having guests? This will be amusing (12)
I believe the answer is:
entertaining
'amusing' is the definition.
(similar in meaning)
'they're having guests? this will be' is the wordplay.
I cannot quite understand how this works, but
'be' could be 'enter' (entering is a kind of being) and 'enter' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for entertaining that I've seen before include "Amusing" , "Being a host - not 4" , "Delightful" , "creating a diversion" , "Considering - holding a party" .)